Manu, 28, joins from Penrith Panthers on a three-year deal, while Hull-born Washbrook returns to the club he left in 2012.
“The fact we have had to fight off competition from a top four club over here for him shows he has come not for financial reasons.
“Washy left the club as a young bloke on the back end of what was a couple of difficult seasons for the club after the 2006 Grand Final.
“He was a fantastic pro when he left and speaking to everybody in and around Wakefield, that’s not changed.
“The competition he’s going to add will be great and the fact that he can play a number of positions gives him that versatility quality as well.”
He said: “I’m really excited about a new start over in Super League and the move was what was best for me and my family.
“I’ve played with a few of the boys who will be at Hull next year before and I’m looking forward to teaming up with them again.”