"Powertech"? Is this possible to make?

Hello. I'm sure you're all familiar with the humans' ability to "powerbuild" with their peasants, by making multiple peasants "repair" on a building that's being constructed, so that it will build faster. So now I need the peasants on the map I'm working on, to be able to make the buildings upgrade faster, when they are told to "repair" a building, or by some alternative method.

I've been trying some stuff out (without almost any progress), but I'm totally clueless. If you have any ideas, please do share them! Thanks for helping out a newbie!

so, you basically want it to work like this:
-building starts upgrade, upgradeing proceeds at normal speeds
-player selects one or more workers and right-clicks on the building
-the workers start "repairing"/"working" on the building, and the upgradeing proceeds at faster speed
is that correct?
well, in that case you will either have to manually make the worker play his build/repair animation, and keep track of all the workers "working" on a building this one via triggers(unitgroups), or you somehow manage the repair ability to work even when a building has full hp. I don't know much about that ability, so I can't tell you if thats possible or not, just play around with it and see if it works.
now, the problem will be changeing the upgrade speed. afaik upgrades have a certain time they need, and that cannot be shortened by any means. if theres really no way to do that, you'll have to trigger the upgrade-process too, which will take away the nice progress bars...

Why don't you just add a small "upgrade" building you put beside the structure being upgraded? You could send additional peasants to build on that side-structure to speed it up. Once the structure finishes, you just replace the main structure by its upgraded version.