They can be plugged from the outside like a light vehicle tire, but most tire shops do not recommend this. The proper repair is to plug the hole and then patch it from the inside or use one of these:
Uni-Seal Plug/Patch Combination Units
I used to plug truck tires all of the time when I managed a drilling rig moving company as sometimes you just don't have time to dismount and "properly" repair a small nail hole. We found that if a plug wasn't going to hold it would fail before we got the tire blown back up to full pressure. So for the relatively light use we put on the drive tires of our coaches I would do it and not worry. Steering tires I would plug to get out of a bind and get properly repaired as soon as possible.