The team did not specify in a statement how many Friday sessions Leimer, winner of the 2013 GP2 support series, would be involved in.
"Fabio will be joining us at selected races for the rest of the season, contributing to engineering meetings and on hand to substitute for either race driver should they be unable to compete at any stage," said principal John Booth ahead of Sunday's Canadian Grand Prix.
"Familiarity with the car is therefore important, so we plan to run Fabio in a number of opening free practice sessions this season, allowing us to further evaluate his potential."
Marussia's current driver line-up is Spanish rookie Roberto Merhi and Britain's Will Stevens, who both bring funding to the tail-end team.
Leimer, 26, competed in the World Endurance Series last season after failing to graduate from GP2 to Formula One.
(Reporting by Alan Baldwin, editing by Peter Rutherford)
