With KiXtart versions 4.10 and above:
Variable with zero-length name ('$') is no longer resolved when used inside strings.

So you could change
Use u: "\\server_6\"+@UserID+"$$"

to this

Use u: "\\server_6\"+@UserID+"$"

But if your not using the NoVarInStrings option this "should" work (but have not tried it)

Use u: "\\server_6\@UserID$"