Merging Old Thunderbird Local Email Into Existing Thunderbird Local

When I had a hard disk crash, I lost a lot of old email, one item of which I really want.  I know that the indices for the various folders are in *.msf files, and the emails in *.sbd files.  How do I import AND MERGE those into my current Thunderbird profile?  I can find import and instructions that don't make much sense for merge.