You can do that with Notepad.
For example, I want to change the MEC Medic kit to look like a US Spec Ops.
Open the kit file in Objects_server/kits/MEC/MEC_Medic.con
bjectTemplate.create Kit MEC_Medic
ObjectTemplate.saveInSeparateFile 1
ObjectTemplate.kitType Medic
ObjectTemplate.aiTemplate Medic
ObjectTemplate.geometry MEC_Kits
ObjectTemplate.geometry.kit 4
ObjectTemplate.geometry.dropGeom 11
ObjectTemplate.setNetworkableInfo KitInfo
ObjectTemplate.hasCollisionPhysics 1
ObjectTemplate.abilityRestoreRate 0.05
ObjectTemplate.abilityInVehicleRadius 7
ObjectTemplate.abilityInVehicleStrength 0.25
ObjectTemplate.abilityInVehicleMaterial 73
ObjectTemplate.unlockIndex 3
ObjectTemplate.cullRadiusScale 2.4
The two bold lines is what you're after.
First one controls which geometry the kit is using, they go from 0 to 9 (max is 16).
Second one controls from which kit.con file the geometry is loaded.
So if I want the MEC medic to look like a US Spec Ops, I need to change the two lines to
ObjectTemplate.geometry US_Kits
ObjectTemplate.geometry.kit 0
You'll need to play around with the number in the second line to find the right geometry, I only know that 0 is the Spec Ops and 7 is Crewman, 8 is Helicopter pilot and 9 is Jet pilot.
Check the faction folder in Kits folder for the name of the _kits.con you want.
If I wanted the Medic to look like a Chinese Spec Op, I'd go to Kits/ch/ and check the name of the file, which would be ch_kits.con.