It's more a model then a cpp issue.
Human models have a number of proxys: One for the riffle, one for rpg's, law's etc., one for the binocular, one for the night vision device, one for the flag you can carry in capture the flag missions and one for the handgun (I hope I have not forgoten anything).
All this is neccessary to get the above mentioned items to show up on the soldier model if used.
All pre-Resistance human models miss the handgun proxy, therefore no handgun will show up if you add it to a pre-Resistance model.
You have to redo your addons with Resistance models or if you have the means to edit the model itself you have to add the pistol proxy.