They have signed the player on a deal until the end of the season – with an option in their favour – after he was released by Toronto Wolfpack on Thursday.
The Championship club only bought Westerman from Warrington Wolves for £130,000 on a lucrative three-year contract in October, but the move has clearly not worked out.
He played only six games for the Canadian outfit.
Westerman made 127 appearances for Hull between 2011 and 2015 before Radford allowed his £150,000 switch to Warrington.
He is now back on a free transfer, but there is plenty of competition for places with FC who twice won the Challenge Cup after the former Castleford Tigers player’s departure.
Radford said “I’m over the moon to be able to bring in a player with Joe’s talent.
“He’s in a position where this has to work for him and for that reason I’m sure he is going to do everything in his power to make this a success.
“Our club has changed significantly since Joe was last here and he is coming into a professional environment where we set very high standards and have a strong culture.
“He understands the challenges that poses and it’s one he is relishing. He provides us with another quality ball-playing option in an already strong pack, with the versatility to play through the middle.
“But he’ll need to get his head down and work hard over the coming weeks to earn his chance for selection.”
Westerman, 28, said: “Radders gave me the call and it was pretty much there and then that I was able to make my decision.
“There were a couple of other clubs interested, but I love this club and that’s why I’m happy to come back here.”
Hull say Westerman had “more lucrative, longer term options on the table”, but his decision to rejoin them shows his desire to prove his worth.
He starts training on Monday, but they do not expect him to challenge for a place for a “few weeks” until he improves his fitness.