Depends on what you consider archiving and what you consider viewable.

For myself I keep 'archived' messages in separate foldes in TB. And of course those are viewable.

When you want to store them on another medium because of lack of diskspace you might export them to .msg on a CD, that way they can be read with TB by double-clicking, but they're not very manageble. Zipped archives of .msg files have the same disadvantage.

Storing messages in TB message base format on CD won't make them very readable, unless you're tricking TB into thinking it has anm account on CD, but that'll bring some problems along.

That's the reason that I stored my keep-mail in dedicated archive folders (I used to have an archive-account, but I quit with that), common folders are a possibility too.