The 36-year-old former Sheffield Eagles and Leeds Rhinos centre has not played for more than a year after undergoing a knee reconstruction.
He left the Rhinos at the end of the 2011 season and was left without a club when Welsh side Crusaders, with whom he had signed a two-year contract, pulled out of Super League.
Senior actually started out in the 15-a-side code with Huddersfield before joining Sheffield Eagles in 1996 - and he now looks set to return to his roots with Leeds, who were relegated from the Premiership just over 12 months ago.
He said: "I would like to thank Chris Gibson and Diccon Edwards for giving me this chance to train with Leeds Carnegie.
"I know they are preparing for a big year in the Championship and hopefully as well as proving my own fitness, I can also help them out given my experience at the top level, albeit in a different code.
"I am still working hard with the medical team here at Leeds and I am confident I will be able to play rugby again."
Leeds Carnegie head coach Diccon Edwards, who had a spell playing rugby league with Castleford, said: "We are delighted to welcome Keith into our group for pre-season training.
"The hope is that he can prove his fitness and we can then sit down with him and discuss the future after that.
"He has been doing a lot of training on his own because of the nature of the injury and it is good that we are able to bring him back into a team environment and I know that is something he is enjoying.
"We have a young squad and his experience is a great addition to our group during this vital part of our preparations."
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