Dear Bernhard,
I'm really thankful for your fast help and implementing the new methods.? But as I wrote, compiling of one file worked like a charm, but to completely customize UIkit like this https://getuikit.com/docs/sass#how-to-build (and any other design framework, too) the import function is - at least in my opinion - necessary. And you wrote, it would need some more work for you to implement it.
So I tried to follow @snck solution (with imports) and read in the SCSSPHP documentation to get it working and maybe do some groundwork for you!??