Ah, that's not something we can change per map at the moment, I'm afraid. Spawn time is just per-gamemode. I also can't honestly think of a convinient way to add that

Alternatively, can you add it in-game? Like how you can adjust the max number of players when you host a game?

Just possibly, but I don't think it would be very convenient even then. I might be wrong, but it seems to me that UE4 just isn't made for storing information about maps like that. I think the UE4 way to do that would be to have a separate game mode with different spawn times, which is obviously getting pretty excessive if you just want to tweak spawn times.
It depends how much you guys want to do this, I imagine I could work something out.

Oh, actually, sorry Rakurai, that was a better idea than I initially thought! Yeah, I can think of a way to do this that wouldn't be too bad now. Is this something you'd want then?