You can add a bone to the model and connect it to a different bone on the player model skeleton. Then, once you create the entity with the model and set its movetype and aiment like this:
Code:
set_pev( iEnt, pev_movetype, MOVETYPE_FOLLOW )
set_pev( iEnt, pev_aiment, id ) // where id is the index of player who should be inside the ball
the ball will be attached to the player, which unlike setting its origin on think, doesn't look laggy.
Check at how Hats do this. It's pretty easy to do, but you need to have basic 3ds max knowledge.