Use a tool like IMGManager or other simiar tool.

1. Import golfani.ifp into gta3.img

2. Either:

   2a. Import multiobj.col into gta3.img. 
       Make sure it is REPLACED and no 2 copies exist!
       Remark: It already contains the volleyball net (od_vbnet) 
       if you have or want to install "SA Volleyball". 
   Or:

   2b. Extract multiobj.col from your gta3.img; 
       use CollEditor 2 to insert kb_golfball.col into multiobj.col; 
       and then import multiobj.col back into gta3.img. 
       Make sure it is replaced and no 2 copies exist!