Technically, it is doable, but (one big BUT) it requires much more effort than it is worth. Currently, the only way to port v7 to our devices is via patchrom (
more info here), a set of their official tools which uses very brute way of patching, and that is by decompiling (reverse engineering) system files, and then compiling them back. On top of it all, the tool which patchrom uses to decompile is apktool (the only tool of that kind that I know of), which in turn doesn't use Java as a lanugage, but a very obscure assembler/disassembler named
smali.
I personally got somewhere to the middle of porting when I decided giving up because there were many patches needed to be done. Since I don't know Java very good, and much more less smali, I decided that I'm not gonna waste my time battling with ~200 patches. It just isn't worth my time and nerves.
I hope, though, that I gave some insight into the process of porting, so maybe someone will be interested in porting miui7 after reading this. Good luck!
maybe I'll deal with it sometime just for fun 
