hello Will and welcome to the forum.
So you want to make a map with peace time borders on in the map editor.
I see 2 ways of doing this.
One is by firing up the random map and then exit and load up the autosaved RNO xxx xxx into the editor and change the map to your likings.
The other way is indeed by using that F5 button, but then it needs to be fixed. I do remember succeeding in getting it to work a year ago when i was trying out things on C I, but to tell you the truth, I can't remember how I did it, if I changed something or if I pressed a hotkey combination to get it to work. I will have a look in my back ups to see if i can find it back.
In any case, you posted this question in the 'modding' section and not the mapping section, so, what you gonna wanna do, is look for the graphic file that holds the peace time border stone ( .gp) and then make sure it shows up properly in the .rsr section file of the F5 menu. (I believe its holes.rsr or that other weird.rsr file)
The problem is, if you mod it, the map will be for single play only AND you cannot distribute the map to other players unless you also distribute a mod.gs1 or patch.gs1 file.
Hope that somewhat helps.
Don't hesitate to ask for more information.