Terminology is critical in IT. For a GDG, both version
are valid terms but mean extremely different things. You said
But, the problem here is, I'm not able to assign different GDG versions to the same DD.
but the context of the reference is generation, not version. This confuses your question, and does not impress responders with your level of understanding.
You can write the SELECT statements for all nine SYSINx files -- the system won't actually attempt to access them until opened. You cannot, however, decide at run time that you need six SELECT statements since the COBOL code must be compiled and the number of SELECT statements will be fixed when the compile is done.