Add an option to Bulk Add to the Affects Version/s field. The current bulk operation will replace the existing selected versions with the version selected in the bulk operation.
Example use case:
Issue A affects versions 1.1, 1.2 and 1.3
Issue B affects versions 1.2 and 1.3
I release version 1.4 which doesn't fix either issue and want to add version 1.4 to the affects field of both issues. I cannot use the Bulk Change Affects Version/s since it would simply set the field to just 1.4 for both issues.