The 27-year-old impressed in a trial spell with the Railwaymen, relegated to League One for next season, and is a free agent having been released by Chester City.
Boss Dario Gradi said: "Ryan has accepted an offer. He looks like he's going to score goals for us and that's been a problem for us in the past.
"There are lots of buts and maybes when it comes to goalscoring but he looks like he definitely will."