How to open official maps in the editor?

[Hacksaw]

I would like to open some of the smaller official maps in the editor to add additional spawn points to make them useable in hotseat. As many of you probably know you can only play 4412-4414 for hotseat as those are the only ones that we know of with multiple player spawns. I am new to the editor and would appreciate a step-by-step for opening an official map and adding in spawns. If all works out I'll fix up some maps for hotseat and make them available to you guys. Any help appreciated! -Hacksaw