I want to point out that every item has by default a BibTeX key even if it does not show as default field in the edit dialog. The optional field you can edit is a special field to override the default key. You probably don’t want to do this for all of your library.
Please also have a look at this Specify custom BibTeX key formats for entire library about BibTeX keys and how we want to improve them (and why we haven’t already…)