I have fields without labels
  z_me51_fld5_1_1
  z_me51_fld5_2_1 and so on
  z_me51_fld5_1_2
  z_me51_fld5_2_2 and so on
I want to define the same domain for all these variables. Is there a "wild card" available to do so?

You can use the LocalValueHelp statement. It can also be used in a loop using a dynamic index to build variable field names.