Waterproof Box THHN Clamp and Interior Run
Instead of using a 1" nipple, use one long enough to reach to the exterior of the adjacent wall.  Then use a proper emt termination connector with a proper emt box.  Or use a emt LB that attaches to the threaded nipple.

Either way, it sounds like your current nipple is too short.

Running MC/AC cable to a threaded nipple won't work, especially if you can't reach it.