First off, subscribe to this addon, allowing you to have an extended properties menu.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=104607712

Then, you walk over to one of the combine's doors, hold C and right click onto the door, then you'll use open/close door to interact with the doors on the properties menu. This works aswell with other doors including locking/unlocking them.

I hope this will help you! ;)