The former Hibernian forward has been training with the part-time side for two months, having been without a club for over a year.
The 30-year-old - who has been linked with Dunfermline Athletic and turned down a move to Arbroath earlier this season - has a formidable reputation as a talented player, but his future in the game remains uncertain. Hartley is unconvinced that Riordan would cope with the demands of the Championship.
"I don't think he is fit enough [to join the club]," said Hartley. "We need a group of players who are going to work hard for us every week and have the fitness levels required for our league. He is working hard and he's a good boy."