I've tried to add mines like that and it did not work. I've also tried loading ships with supply so they start loaded and other stuff that you can do in game without success.

I do not know how it is done officially but I'm convinced the editor is incapable of doing everything you need for ...

1. Get the device number of the mine you want to have in the Devices tab.
2. Go to the Locations tab and find the base you want the mines to be at.
3. In an unused Weapons slot, put in the mine device number and then the number of mines you want to be there. E.G. the Mk 6 mine is device number 123 ...

How to you place mines in ports with the editor?