Can this panel have a main shutoff switch installed on it?
You could install a main breaker (what you're calling a main shutoff) but that REALLY shouldn't be done live, by anyone.  You'd have to lift your feeders and land them on the new main breaker terminals.
When the disconnect is not where you can keep an eye on it while you're working, like in your building, it should be lockable.  If the disconnect has a one-arm-bandit type arm to open it, those can be locked open with a padlock.  If it's just a regular breaker, you might be able to find a breaker lock that clamps on the breaker handle and can be locked with a padlock.