If properly engineered and designed, one may run the above DG sets in parallel. They have to be properly:
1. System grounded (system ungrounded is possible but to be avoided, if possible)
2. Individually protected
3. The downstream power distribution must withstand their total short-circuit fault currents and short-circuit MVAs.
4. Designed for the same frequency
5. Avoid a short intermittent on-off duty of one of them to keep system stable.
6. Loaded according to their permissible loading, kVA vs time curves.

In addition to the general factors mentioned by jbartos, you need the following specifics-
1. The governors must be set up for parallel running.  This usually means that they must be set for the same percentage droop, so that they can share load in proportion to their ratings.  Other control options are available, depending on the type of governor in use.
2. The voltage regulators must also be set up to share reactive load in proportion to rating.  This will usually also mean load droop control, or cross-current compensation between the two AVRs.
3. Of course, you will need synchronizing instrumentation & controls to parallel the units in the first place.  This can be automatic or manual & can be simple or complex depending on your requirements.

Thanks to jbartos and peterb for their helpful hints. The system where I am going to implement this is already running 3 sets of 500 KVA in parallel. The neutral (star point) of each alternator is connected to ground through an isolating contactor, only one of which is on at a time
(the rest being floating).The latest set of 180 KVA is being added to take care of peak load requirements.
The paralleling instruments already in place are : Synchroscope (rotating LED type), Check Synchronising Relay(SKE11 Electro-mechanical)and also the dark lamp method! The other protections are Reverse Power Relay(CCUM21) and Earth Fault Relay(CAG14)for each DG set.Each Alternator is protected by motorised air circuit breakers.
I was wondering if you know any reference book on this subject of synchronising of DG sets.  

As far as references are concerned, I suggest that you try the appropriate manufacturer websites.  Basler and Woodward governor would be good starting points.

Suggestions:
1. I had to keep my answers in general to avoid any unrelated specifics such as the power management, loading percentages, etc.
2. The original posting calls for safe operation of the additional 180kW DG. The power management strategies are considered on safe side.
3. More info on DG units and their controls is available over manufacturers:
http://www.thomasregister.com
3a. Type Governor under Product / Service that will return Governors: Diesel Engines and 2 Companies in addition to the mentioned ones in the previous posting.
3b. Type Generator Product / Service that will return Generator Sets: Diesel Electric 181 Companies
for good selections on more info
4. Reference:
Bergen A. R. "Power System Analysis," Prentice-Hall, Inc., 1986 Page 390 Section 12.5 Special Case: Two Generator Units describes in more detail "power control of two generating units," including frequency droop characteristics.

Even as we were corresponding thru Eng-Tips,I was carrying out the installation work (cabling, control wiring etc.)at the site. Yesterday, I commissioned the set (180 KVA, 415V, 3 phase) by synchronizing it with the existing 3 x 500 KVA sets. I am glad to report that the set ran for over 4 hours in a synchronised state without ever tripping. As the load sharing is being done manually, I tried to load the 180 KVA set but could raise it to only to 90 KW. At this loading, it was drawing a current of 200 Amps. This ampere load remained fairly constant even as I decreased the KW load even to 20kW ! I am still scratching my head over that !
I have asked for the governer to be calliberated afresh
( mainly to give me time to think of a viable load sharing scheme ! )
Well, thanks to jbartos & peterb for their interest and without whose prompt replies, I would be still groping in the dark.

Sunilji -
Glad to hear that you were successful in commissioning the set.  A couple of thoughts re your results -
- At 90 kW, 200A, 415V you were operating at 144 kVA and a power factor of 0.63; reducing the real power to 20 kW, you kept at 200A with a power factor of 0.14.  How is the control mode of AVR set up?  
- You may want to try reducing the excitation to increase the power factor up to the rated value (0.80? 0.85?) for this loading.  How did the bus voltage respond when the unit came on line?  It should have increased noticeably, so that reducing the unit excitation would bring it back towards nominal.
- Assuming that you are not connected to the grid, you may have to back load off of one of the other running sets to get the 180 kVA unit to pick up more load - it all depends on the droop settings. What control mode is the governor set up for?  You may also want to verify that the prime mover isn't limiting the output due to some mechanical problem.

Suggestions:
1. Power flow simulation by suitable software performing power flow analysis/management could help.
2. 180 kVA generator is supplying power to the parallel generators. Please, notice that various regulators, controllers, governors, etc. have their boundary conditions and operating regions. If the current setup does not fit the limiting conditions, there may be a need for their customizations.

Suggestion: Visit
http://www.dynex.com/fyi3.htm
http://www.egsa.org/powerline/past/mj99plantbas.htm
for more info, and
Selco USA, Inc. for Load Sharer T4300 for application diagram at 770-455-9110 (Atlanta, GA, USA) or email SELCOUSA@worldnet.att.net