I wouldn't. Infact i'd do my best not to feed anything aside from the pellet, until he accept's them readily. I found out first hand, that some individual's may simply allow themselves to become emaciated rather than eat a pellet.
I too would be inclined to querey what the past owner had been feeding.
Update: He is now greedily scarfing down the Hikari Gold and Staple. Thanks for the tips. I held fast and he gave in...now I'll let him have some krill.
My escondido wouldn't eat much of anything for over a week. He decided omega one flakes are edible, however, so he gobbles those up. He doesn't eat pellets though.