Hi Nina...
It sounds like you should look in your EDD. An attribute definition can specify that the attribute's values are of type "Choice", in which case there will be a "Choices" element that specifies the valid values. You can modify that list of valid values, but if you do so, you'll alos need to update that same list in the DTD.
Note that if you are using DITA, you can't just change these lists and remain compliant with the DITA specification. That's not really a big deal unless you expect to be sharing your files with other people. When someone else tries to open your file that contains an enumerated value that's not in the DITA spec, they will get an error.
Cheers,
...scott