Maybe send the Drillers to a City where they can be repurposed over a normal production cycle (8h isn't it) and turned into Builders. Then the builders (who have no combat power) can convert a factory into a Generator, or a destroyed outpost into a Factory. Make it progressive, Generators can only be built from a Factory. Require a significant number of them. Drillers need to sit at a City for 8 hours after repurposing is selected before they become Builders.