Hi Tobias,
Currently there are only regular sub/super scripts supported. However, you could just do a subscript on a space (or invisible space) then U followed by subscript 2. One could tune the spacing between the two subscript using negative and positive spaces:
"\!\(\*SubscriptBox[\(\[InvisibleSpace]\), \(12\)]\)\[NegativeVeryThinSpace]\!\(\*SubscriptBox[\(U\), \(34\)]\)"
Where do you want to use it? Inside a table? inside a graphics? or ...?