Returning from last season: Matt Clapp (he was with the Lions for part of training camp)
Currently a free agent: Jerome Felton (he is a restricted free agent; the Lions tendered him at the original-round level), Jake Nordin (he was on injured reserve last season and is currently an exclusive-rights free agent)
New to the team: None
The last few years, the Detroit Lions' fullback position has essentially been only Jerome Felton. Going forward, that very well may still be the case, as Felton was tendered at an original-round level by the Lions earlier this offseason. Felton has previously said that he wants to be used more or play elsewhere, but since he is a restricted free agent and has been tendered an offer, it's likely that he will be back.
The other two fullbacks currently in the mix are Jake Nordin and Matt Clapp. Nordin converted from tight end to fullback last year before suffering an injury in training camp that landed him on injured reserve. Clapp was let go during training camp, but the Lions re-signed him at the beginning of this past February.
Need going forward?
Unless Felton were to sign somewhere else or be traded, I can't see the Lions adding another fullback that would compete for a spot on the roster. It wouldn't surprise me if an undrafted free agent fullback is brought in as a training camp body, but with three fullbacks already expected to be on the roster, there isn't much of a need at this position.