I can't find where you paint creeper onto the map
that's because you can't yet, it's intended for a future update i believe, but V has things to do before that so don't hold your breath ;)
so if you want C on a map you can place emitters, and AC emitters to keep areas clear, and then run the game for a bit and delete the AC in the editor terrain panel.
presto a map with C and a clear zone, but no AC.
hope that helps 8)
If you really want some creeper at the start of your map, use CRPL (you can even use CRPL to render the second frame so it shows up (unconfirmed))
Yes, use a CRPL core with a simple SetCreeper and then call destroy on the unit.