Make an entity then change the classname key to weapon_railgun or weapon_bfg or whatever. Look up the classnames on RUST or some other Q2 mapping site.
edit: Oh, a compiled .bsp? Can do this a variety of ways such as decompiling and placing an entity in BSP, then recompile the new map to bsp, then open it in notepad, scroll to the end, and copy all of the plain texty stuff out and into a file which you call mapname.ent and then put it in your pball/maps/ents folder
