If you are using kN & m, the units of mass will be metric tonnes (1000 kg). So if you have a weight of 100kN, the mass will be 100/9.8 = 10.2t.
The rule for the units of mass is you take your units of weight, and divide by the acceleration due to gravity in the units of length you are using...sometimes this doesn't come out to a normal unit (for instance if you use kips and inches: (kip.s^2/in)).