You can reassign an unused key to function as u using an external program such as SharpKeys as listed here, which works globally across Windows (there's a way to do this through registry editing, which is basically what that program does for you), or you can use AutoHotkey if you want a key combination to do this.
The advantage of the former is you don't need to run anything; the operating system does it for you. It basically turns your assigned key to u. I did this once when one of my keys stopped working (I sacrificed my seldom-used Menu key between the right-hand Ctrl and Alt).
The advantage of the latter is, as mentioned, that it allows key combinations.
Here's an example for an AutoHotkey script that makes the Alt+1 key combination behave like the u key:
And one for Ctrl+4:
You can find more information on the various keys and modifiers in the AutoHotkey documentation (I'd put links, but my reputation isn't high enough for that apparently).