If I understand your question, you can export your contacts list from whatever program you have them currently stored in, and import them into Thunderbird. If you have a choice between VCF (vCard) format and CSV (Comma Separated Values) formats, I recommend using vCard (VCF) - less fiddly and error-prone.

As far as I can tell, you can only import CSV contact lists into Yahoo, and you must use the old web mail interface. The "new" interface doesn't have the option of importing a file.

If there's a way to sync contacts between Thunderbird and Yahoo, I haven't discovered it yet.

Search web for "does yahoo support carddav thunderbird"

When I try to add a CardDAV contact list to Thunderbird's address book, I get the error message, "yahoo.com is known to be incompatible with Thunderbird." There's a "Continue" button, but it does nothing.