Foster joined the Airlie Birds on an initial one-month loan deal after being unable to regain his place in the Saints team and has played in Hull's past five matches, scoring one try and 21 goals.
He will now be available for selection for the Black and Whites until he is recalled by his parent club.
Meanwhile, Hull are anxiously awaiting the results of scans on utility player Richard Whiting and winger Tom Lineham.
Whiting picked up a foot injury in his side's 30-10 win over Catalan Dragons earlier this month while Lineham was carried off with concussion during last Saturday's defeat at Widnes and has gone for x-rays amid fears he may have broken his ankle.
- Richard Whiting