I have successfully done that with special weapons such as diamondbacks and Guardian.
In the weapon_equip.ini file, find the gun you wish to edit
[Gun
nickname = special_gun07
ids_name = 263542
ids_info = 264542
DA_archetype = equipment\models\weapons\co_proton_cooker.cmp
material_library = equipment\models\ge_equip.mat
HP_child = HPConnect
hit_pts = 6752
explosion_resistance = 1.000000
debris_type = debris_normal
parent_impulse = 20
child_impulse = 80
volume = 0.000000
mass = 10
hp_gun_type = hp_gun_special_10 <<<<<<< change the number here
The number determine the class of the weapon, you can change it from 1-10. If you delete it, it will fit into any weapon hardpoints except CM and Mines.