Expression names to be used in UDF

Hello all;
I am curretly working with expressions and UDF in NX 9:
My Problem statemenT includes expression and UDF: I wish to use self defined expressions in UDF as an option
Expression:
a=8
b=9
c=10
d=11
Diameter=a or b or c or d
In UDF I wish to give these predefined expression as options.
Is it possible to give these expression names as option in UDF?
e.g
UDF:
Diameter: a
b
c
d