Titiriga,
I agree with Aaron that you should create Properties and here is the reason why:
It is possible to modify that list of available values but you would have to modify its GDL script. To do that, you would probably have to extract the LCF (Library Container File) containing the Archicad default library, find the library part or macro where those values are defined, modify them, save them as new library parts, (optionally) replace the default library parts with the modified ones and recreate the LCF. Then, you would have to do this again each time there is a Library update.
So the Property way is better, in my opinion.
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