Currently you can't. It may seem ridiculous but it's actually a more complicated problem than other games have to deal with because of chronocam.
If you wonder what chonocam has to do with it, the goal is to be able to take over a replay from any point. (So replays become savegames)
A lot of data are missing from replays. For now, you could only start a new match with a bunch of existing units from a replay (and I don't know if the AI would break or not).
