I assume you want to add a handgun to a non-resistance unit, but while still running OFP: Resistance. Is this correct?
If not, then you cannot use handguns without running OFP:R.
If that is correct, then you add the handgun just as you do any other weapon, with the addWeapon and addMagazine commands.
Let's say you have a unit that you want to give a Beretta M9. You would add the following code to the unit's initialization field:
this addMagazine "berettamag"; this addMagazine "berettamag"; this addMagazine "berettamag"; this addMagazine "berettamag"; this addWeapon "beretta"
This gives him 4 M9 magazines, and the M9 pistol. All you need to know is the name of the pistol and magazine, and you can add the weapon just as you would a primary weapon.