The latest Xbox Live Marketplace schedule lists Insanely Twisted Shadow Planet expansion Shadow Hunters for release on 13th September.
It's priced at 400 Microsoft Points (about Ł3.40).
The XBLA add-on is expected to add the co-operative mode developer FuelCell previously revealed to Eurogamer.
"One of the things - while we're really happy with how [existing multiplayer mode] Lantern Run turned out - we wanted to bring a co-op experience that's a little bit more like the core campaign experience," FuelCell CEO Joe Olson explained. "That's kind of our goal with the DLC."
A 2D side-scrolling adventure akin to early Metroid and Castlevania titles, Insanely Twisted Shadow Planet launched last month as part of Microsoft's Summer of Arcade promotion.
Eurogamer's Simon Parkin rated the game 6/10, calling it "the nucleus of a Metroidvania game, mechanically functional and regularly interesting, but a shadow of its inspiration nonetheless," in his Insanely Twisted Shadow Planet review.