Is the pneumatic controller local or in a control room?  If control room is it two pipe or four pipe?

http://www.scanivalve.com/ Scanivalve used to make a pneumatic multiplexer to sample the pneumatic signals in an old control room.  This may still be available.  I would rather replace everything with new electronic transmitters, DCS IO and digital valve positioners.  Electronic transmitters have lots more precision (thus greater turndown) and are rugged and reliable.  Consider replacing the pneumatic controllers.  You could add a pressure transmitter on the pneumatic signal if the pneumatic controller uses a pneumatic transmitter instead of process connected.  I would prefer a new electronic process transmitter wired to the DCS.

I completely agree with JLSeagull.

Change to electronic tranmsiiters and valve positioner.

I trust this helps,
Mlv
http://www.coulton.com

A P/I pressure to current transducer will do the job.  I'm assuming you are trying to just indicate the measured variable to the DCS?

These were/are very common where a lot of pneumatic plant is gradually converted to DCS, or to avoid conversion in hazardous areas.
 

I had a local company build us a cabinet with P/I and I/P's put in the cabinet.We mounted it next to the new PLC and tubed the signals in and out from the old pnematic memonic board.  It worked perfectly.  We had the deltaV screens redraw to look just like the old board and the operators didn't miss a lick.

It was 1/10 the cost in 1/3 the time.  The cut over was done on the fly, one loop at a time.  No need for postioners as the valves and the loops were already tuned and calibrated.