All your add-ons are stored in the SITE_ROOT/packages folder so you could theoretically modify any file you want.

That being said, I'm sure many add-on developers would cringe at the thought of anyone modifying their add-on then asking for support if/when something goes wrong.

My advice would be to contact the author FIRST and see if what you're doing can be done (or will be done in the near future), or if they're nice enough AND the modification is minor to simply give you the supplemental code themselves.

Forgive me if you're already an experienced add-on dev, but simply adding form fields in the block's add/edit files will not make the block save said data. Each block typically has a database table associated with it that corresponds in some shape or form to the fields on the block's add/edit window. Any changes to one should be reflected in the other and can be rather complex (depending on what type of user-inputted data is being added).