Briton Bearman is set to race for Haas full-time next season in an all-new lineup with Frenchman Esteban Ocon and made his F1 debut for Ferrari in March when he finished seventh as a stand-in for appendicitis-stricken Spaniard Carlos Sainz.
Danish driver Magnussen triggered a mandatory one-race ban at last weekend's Italian Grand Prix after collecting 12 penalty points in a 12-month period on his super licence. He will return in Singapore with a clean licence.
"It’s definitely more of a challenge stepping in to race as a reserve driver, with limited prep-time and so on, but I’m in the fortunate position of having done it earlier in the year... so I can at least call on that experience," said Bearman.
"I’ve also had four FP1 (Friday practice) sessions (with Haas) in the VF-24 already this season, so undoubtedly that will also prove to be valuable in tackling the full race weekend in Baku," added the 19-year-old.
Haas are seventh in the standings, six points behind Red Bull-owned RB and 15 ahead of Renault-owned Alpine.
"I’m excited that Ollie will be driving the VF-24 alongside Nico in Baku," said team boss Ayao Komatsu.
"This is another excellent opportunity for both Ollie and the team to work together, this time throughout an entire race weekend, and he couldn’t ask for a better teammate than Nico to provide him with a reference."
