Ngata, who was traded to Detroit by the Ravens in the offseason, can come off the list at any time. The team said they expected Ngata to return to action in about a week.
Seisay played in 13 games for the Lions last season and produced five tackles.
Bell is on the Physically Unable to Perform List after missing all of the team's offseason practices due to the surgeries.
Bell sat out all offseason practices after undergoing surgery for an issue that he has yet to divulge. But Bell did have problems with his knee and Achilles last year.
He should be set as the No. 1 option out of the backfield in 2015 after the release of Reggie Bush, though rookie second-round pick Ameer Abdullah could push him.
Stanford was one of five cornerbacks that worked out for the Lions on Tuesday. He has two interceptions during his five-year career.