System variables in InputSets: Is it possible to use GuiXT system variables in InputSets?
No, this is not supported. You can use a workaround calling up an InputScript from your InputSet. Example:

[X]
OK:,process="myinputscriptX.txt"

// InputScript myinputscriptX.txt
Set F[VIQMEL-BSTNK] "&V[_user]"
Set F[VIQMEL-BSTDK] "&V[_today]"
Enter "+U01"