I've tried selecting a unit, I've tried moving it into another empire's Outpost during open-borders, even *on top of* their Outpost; no matter what, hovering over the action-button in the top-left simply yields:

Cannot be done as there's no valid tile

I had to put my unit on the tile of a city/center and then pay the money to have them slowly steal a city from a player I am at peace with.