The 23-year-old joined Livingston on a deal until the end of 2015-16 after leaving Dundee United in January.
He previously spent the first part of last season on loan at Almondvale Stadium, and made a total of 30 appearances for Livi last term.
McCallum, who has also played for Arbroath and Berwick, is the League Two side's 14th signing of the summer.
Find all the latest football transfers on our dedicated page.

Their lab-based work suggests there is a biological step that can restore normality and stop cells replicating out of control.
When the US researchers added molecules called microRNAs, it put the brakes on cancer, Nature Cell Biology reports.
Although the early tests in a dish look promising, it is unclear whether it will help treat people with cancer.
The Mayo Clinic researchers are hopeful that the new mechanism they have found could apply to all types of cancer.
It brings together two strands of science - cell adhesion and microRNA (miRNA) biology - that, until now, had not been linked.
Scientists had thought adhesion molecules were simply the glue that holds cells together. But then others found they might have signalling roles.
The Mayo Clinic work shows adhesion molecules connect cells and signal via miRNAs to control cell growth.
If this becomes deregulated it can drive cancer.
But replenishing cells with miRNAs can remedy this.
Lead researcher Dr Panos Anastasiadis said: "By administering the affected miRNAs in cancer cells to restore their normal levels, we should be able to re-establish the brakes and restore normal cell function.
"Initial experiments in some aggressive types of cancer are indeed very promising."
Henry Scowcroft, Cancer Research UK's senior science information manager, said: "This important study solves a long-standing biological mystery, but we mustn't get ahead of ourselves.
"There's a long way to go before we know whether these findings, in cells grown in a laboratory, will help treat people with cancer. But it's a significant step forward in understanding how certain cells in our body know when to grow, and when to stop. Understanding these key concepts is crucial to help continue the encouraging progress against cancer we've seen in recent years."

Mr Deby took power in a coup in 1990 and later introduced elections.
A referendum in 2005 scrapped a clause restricting presidents to two terms, but Mr Deby said that if he gets re-elected he will reinstate it.
In the last year, Burundi, Rwanda and Congo-Brazzaville have all changed their constitutions to allow their presidents to run for office again.
Africa Live: BBC news updates
Mr Deby said on Tuesday that reinstating term limits would provide "vitality" to the democracy.
"We must limit terms, we must not concentrate on a system in which a change in power becomes difficult," he told the ruling party.
"In 2005 the constitutional reform was conducted in a context where the life of the nation was in danger," he added.
Africa's longest-serving leaders:
The arrogance of power
The Mobutu and Gaddafi effect
Plymouth Argyle have signed former Livingston goalkeeper Marc McCallum on an undisclosed-length contract.
[NEXT_CONCEPT]
Scientists believe they may have found a way to turn cancerous cells back into healthy tissue.
[NEXT_CONCEPT]
Chad's President Idriss Deby has announced he will run for a fifth term in April's elections.