Best way to convert from IMAP to POP3?
I've not used Thunderbird but I assume similar functions (multiple accounts, local folders) are available to you.
This is how I would do it, based on Outlook:
Set up the POP3 account alongside the IMAP account
Set up an empty folder structure in your local folders (equivalent of Outlook's PST file)
Drag and drop from the IMAP folders to the local folders
Whilst it is not a "one click" solution it will save you from moving all to the inbox and downloading via POP.