This is a little complex, but I'm going to try to condense it.
The Author field is a multi-value field, which means that you can have more than one Author per file. Now, when you add an Author to this field, MC allows you to select that Author on any other file with the drop down list. So, if you were to create a file that had, for example, 250 Authors for THAT ONE FILE, then all 250 authors would become available in the drop down list in any other file. So the trick here is to get all of your Authors entered into one or more files in MC. Than you can choose those authors for any file you have.
If that result would be good for you, then you just need to get one file's Author tag populated. Probably the best way to do this is to create an MPL file. That's an MC specific XML file format which can take many values for each file. You would want to choose an existing file, and then "dump" all of your Author data into that one file as a place holder. You would need to create an MPL file with all of these values. I've messed with MPL some, but I'm not an expert and I can't give you direct guidance on creating this MPL file. But it's not that hard. Do a little research and an experiment or two.
Once you have created your MPL file, you need to import it with File > Import Playlist . NOTE: With MPL *sometimes* you need to import it TWICE before everything shows up. Not 100% sure why, but I've experienced it for sure.
So that's one way to do it. There are probably others.
Good luck.
Brian.