Haslam joins Scott Redding on one of the German marque's two factory M1000RR machines in place of Sykes, who in turn had been standing in for injured full-time rider Michael van der Mark.
Sykes was involved in a heavy crash in the final race earlier this month at Donington, getting tangled up with the Ducati of Michael Ruben Rinaldi and Bonovo BMW rider Loris Baz at Coppice corner.
While Baz escaped uninjured, Sykes was taken to hospital and was diagnosed with 10 broken ribs and a fractured left ankle. Rinaldi meanwhile was taken to the medical centre but was cleared of any major injury.
Sykes left hospital on Sunday but remains unfit to ride this weekend at Imola, while van der Mark - who has been sidelined since breaking his left femur in a crash at Assen - is also still not ready to make a comeback.
As such, BMW has elected to call up Haslam, who is representing the brand in British Superbike this year. Imola will mark his first WSBK outing since last year's Portimao round, where he rode for the Pedercini Kawasaki team.
"First of all, many thanks to Leon for being available at short notice to be with us at Imola," said BMW motorsport director Marc Bongers.
"We know each other well from the past, where we celebrated success together, and this season he has returned to the BMW Motorrad family in BSB and is familiar with the new BMW M1000RR. He also has a lot of WorldSBK experience, and we are confident that he will quickly get back up to speed at Imola.
"After his crash at Donington, Tom Sykes is on the road to recovery and was discharged from the hospital on Sunday after a week, and we all send him our best wishes.
"Michael participated in a track day last week, however a return for Imola would still be too early, and we have jointly decided that he will sit out this weekend, and then we will reassess the situation."
Haslam previously rode for BMW in WSBK in 2011-12, scoring a best finish of fifth in the championship.
The 40-year-old sits fifth in the current BSB standings with six podium finishes to his credit across the opening five rounds.
“It’s a fantastic opportunity," said Haslam of his WSBK return. "I have a lot of history from this circuit and it’s fantastic to ride again for BMW Motorrad Motorsport.
"We are riding the BMW M1000RR in the UK so I’m looking forward to work with [team boss] Shaun Muir and all the guys from the ROKiT BMW Motorrad WorldSBK Team.
"We will see. I have no real big expectations, I just want to try to enjoy and hopefully we can get some good information and some good results.”
