OK so, there are two options here: one is to just AU the Les Miserables characters forward into the Martian's space program where they would fit in just fine, but that feels like cheating?

The other option is as a sequel to my ever-researched never-written canon-era AU where Les Amis survive the barricade but end up in prison, from which, a few years later, an also-not-actually-dead-but-also-in-prison Galois recruits Combeferre for an *intensely* steampunk, secret balloon-based one-way moonshot as part of a France/England space race triggered by the not-actually-a-hoax events of the Great Moon Hoax, and of course Combeferre gets his friends recruited too. Upon arrival on said satellite they of course swiftly join in with Moon Rebels to overthrow the Man-Bat royalty of the moon and settle there in a new democratic-socialist-Fourierist utopia.

Anyway that's all backstory to the crossover, because of course eating Moon Food prevents human aging so as soon as their telescopes pick up Mark Watney's surival Les Amis arrive over Mars in a moon-zeppelin to rescue him. He reaction is basically 'sure, this might as well happen'. He introduces disco to Moon society and it's all downhill from there.