The Leeds Rhinos legend, 44, has been out of the professional sport since leaving the Widnes Vikings head coach role in 2018.
Cummins and Poching played together at Leeds Rhinos where they also both started their coaching careers together as assistants.
Cummins was Bulls head coach between 2012 and 2014 and then had spells in rugby union as an assistant at Yorkshire Carnegie and Doncaster Knights.
Poching - who claimed the Belle Vue job after winning five of his seven games in charge after taking over from Chris Chester in August - has also promoted Mark Applegarth to be his other assistant.
The former Wakefield player has been Trinity’s head of youth for the last four years but will step up with current assistant Andy Last having already been reported to be joining Castleford Tigers next term.
Poching said: “I am extremely happy to be able to bring Franny back into the game and to have someone of his experience and knowledge to support myself and the team.
“Having worked with him, I know all the qualities he will add to our team and the club as a whole.
“It also gives me great pleasure to be able to promote Mark Applegarth from within the club and to show our commitment to having a coaching pathway.
“He brings a wealth of knowledge of our junior players and club systems and a hunger to work in coaching at the top level.”
Cummins said: “I’m really looking forward to joining up with Willie and Wakefield Trinity.
“Willie and I worked really well together during our time at Leeds and we have always had a similar approach to how we want the game to be played.
“I’m now just really excited to getting things started with Willie, Mark Applegarth and the rest of the staff, and I’m looking forward to contributing to Willie’s coaching provisions and to continue Wakefield’s improvement.”
Applegarth, 36, said: “It will be fantastic to work with Willie and Francis and I can’t wait to get started.
“I was helping out a little last year and it will now be great to make that next step and coach with the squad full time.
“I will continue to oversee the youth system at the club along with Roger Lambert, but my main coaching will now be with the first team and it’s something I’m really excited about.
“Willie is a genuine person who I have known since I was a kid at the club.
“He has great values and really cares about the players he works with. It will be great working with him and learning about the man-management of senior professionals and getting the best out of them.”