Last year, we announced an exciting improvement in Blogger blogs - the ability to export and import blog content (comments / posts) between Blogger blogs (as long as you have administrative access to both blogs). At the time, that was interesting, but not so useful, as only Blogger blog content was supported by the process.
Last month, Google announced an improvement on that possibility - the ability to export and import blog content between Blogger, LiveJournal, MovableType, and WordPress. Hopefully, other blogging platforms will follow in availability. It's possible that both the receiving and sending platforms will have to make adjustments to their code, to enable additional platforms to be included.
With a blog of any significant size, it would be wise to import, then publish, in two separate steps. Get everything imported, and check out the Posts list under "Imported", before Publishing.