I'm a bit surprised to learn that they haven't improved hardly anything when it comes to the migration or conversion process.
Because although I'm currently very happy with my Outlook 2010 environment I still shudder at the actual process of converting my Thunderbird environment to Outlook 2010, that wasn't easy at all.
And now it seems the same applies to 365, which I honestly consider to be a failure. Especially if you take into consideration that popular software such as Thunderbird is open source, therefore people and companies can easily check the way it stores it's e-mail. Better yet: even if you don't want to, the format used is (to my understanding) very similar to the mbox format, with the exception that they're also using an index or database.
But no... As you said; Outlook 2010 can handle IMAP, but you can't simply tell it to import your Thunderbird settings or point it to an mbox format mailbox and tell it to convert this to it's native PST format.
And yes; I know that there are a dozen software packages out there which can do this for you, that's really not the point here. In my opinion Microsoft should have implemented some sort of solution themselves as well. At the very least to make it easier (and thus more appealing!) to convert! At least they should have created some kind of PowerShell script.
Either way; as said I still remember my conversion. 5 different e-mail accounts; 1 business, 2 Hotmail accounts (business & private), one private blog account and the account provided by my Internet Provider (which I use the least). I ended up exporting all the e-mail from ThunderBird to mbox, uploaded that to my mailserver and with a nice Perl script (mboxtomaildir) I could implement the whole thing and make it accessible using Dovecot.
Then I setup an account to point Outlook to this temporary mail account and within Outlook moved all my e-mail messages from this place to the official account I setup.
So picture my surprise to read that basically Microsoft hasn't changed a thing with their latest 365 environment. This is really not the way to motivate people to start using Outlook. In fact; the process alone has often made me advice ThunderBird to some of my customers. Simply because moving to an Outlook environment would at least take one hour for the conversion itself (probably a little less, but I normally charge by the hour).