ConVars

Set the maximum number of entities a player can hold

sm_pickup_max

Drop all carrying entities on death

sm_pickup_drop 0/1

Admin Commands

Use while looking at the entity you want to pickup to pick it up

sm_pickup

Limitations

I have tried parenting the entity to the back of the player but it becomes invisible. Therefore, it currently moves frame by frame which causes a "laggy" entity moving behaviour.