AFAIK you can't have soldiers pick up pre-placed mines (mines placed in the editor) because you need the person who laid the mine down
Then I would suggest naming your mines and making some script that makes your AI "engineers" walk to them.. and then the mines are setpositioned to some other spot on the island.
That would be about the only way to do this that I can see based on your wanting to use pre-placed mines from the editor