Is adding fish possible?

Puffers are well known to bite and har tank-mates. Mollies are normally top to mid tank schooling fish and should occupy most of their time right in the middle of the tank in plain sight. Yours is probably hiding because it is being singled out and bullied by the puffer. More mollies might not help if the puffer has it in for this particular fish. I'd say give it a try and if it doesn't work consider a different combination of fishes.