as I said, it's only if the upgrades is of one type...
example:
a successive damage upgrade...
or if the effects of the upgrade is applied via triggers (meaning the upgrade is just actually a dummy), you can also use that method...
if not, there's no possible way to directly add a new upgrade to a unit/building... so you will need to replace the building...
or use dummy abilities for the visible upgrade button, then just trigger the upgrade itself... though you won't be able to see the upgrade bar using this method...
or if you need the bar, just create one upgrade (this will be a dummy), add some levels to it... then use that upgrade, now when the upgrade finishes, use triggers to set the level of the actual (real) upgrade to the desired level...