The 22-year-old Belgian was released by Leicester City at the end of last season but impressed in a loan spell at League One side Chesterfield where he netted 4 goals in 10 appearances.
Moussa was part of a Nottingham Forest side which also contained new signings Danny Collins and Greg Halford and fellow trialist Ismael Bouzid.
"We had Franck on loan at Doncaster and he has been at Leicester, so we were aware of him and what he has to offer," said boss Sean O'Driscoll.
The Reds delayed start to pre-season was evident in their 2-1 defeat, Ishmael Miller scoring the visitors goal.