UPDATE: Turns out this build is an outdated build of the version that does not support mods. We will update this version ASAP.
Even so, It also seems that quicksave already works in Skirmish mode in this uploaded version. I think how it works is that it only allows quicksaving for as long as you have not quit the level, but a trick to that can be to just quickload as quickly as possible when the skirmish level begins to workaround this. Because of how short these missions tend to be, we did not bother with a full-fledged quicksave feature for Skirmish. It felt superfluous.