We used to have the ability to search for multiple part numbers in the 'Insert Existing' window from the part structure tab. In 9.1 we were able to enter multiple part numbers separated by semicolon and the results would be returned for all the parts given. This feature was removed in 10+, but is was possible to create a customization to add the feature. In 11.1, it appears that the product structure editor architecture has changed quite a bit and it looks to be extremely difficult to implement such a customization. It would be great to have this functionality in the OOTB product.