Given that, it might make more sense to divide up the transfer into two
parts. Icedove is really a rebranded version of Thunderbird. So perhaps first
you'd go back to your Mac and make the transition from Apple's Mail.app to
Thunderbird. That would handle the conversion. Then, the copying of the
Thunderbird stuff over to Icedove should be easier since the harder part of
doing the translating will have already been done.
Once Thunderbird is installed on your Mac it should be possible to go to the
Tools menu and select Import and get your stuff from Apple's Mail program.
Once you're that's done and you're ready to go to Trisquel, see the "Manually
moving files" part of "Moving Thunderbird Data to a New Computer" at
https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer
