AUX file import - use multiple libraries

In Tools -> New sublibrary based on AUX file… accepts only one reference library. However, I use multiple bib files for a project, therefore it would be useful to accept multiple reference libraries. Specifically, I put the references into several bib files (logically where they belong to) and use some of them when writing a paper. However, the publisher needs a single bib file with no “unnecessary” entries. Currently, I have to do the procedure with Jabref for each library and then manually copying the contents of the “reduced” libraries to a single bib file.