When MT runs it's archiving it puts everything inside an <MTBlogArchiveURL> subdirectory. When the archiving rebuilds are done they base themselves on this path. You can move 'away' from that path using '../' directory traversing.

A default daily template goes to "<$MTBlogArchiveURL$>year_mo_day.html". You can create a new file template to use subdirectories. I use this for my monthly archives: "<$MTArchiveDate format="%Y/%m/%m.html"$>". This dumps files to my site using "<$MTBlogArchiveURL$>/year/month/month.html" locations. It makes for a little tidier processing of directories. I sync using unison, but that's a whole other discussion.

What you may not realize is it's also possible to use a template like: "../other/<$MTArchiveDate format="%Y/%m/%d.html"$>". This would move 'up' the directory hierarchy and then down into the 'other' directory. If you wanted to have MT create the same URLs for daily archives as Radio you'd use this for a Daily Archive file template:

../<$MTArchiveDate format="%Y/%m/%d.html"$>

This would ascend up from the usual /archives directory and then write daily files into new year/month subdirectories.

You can use this in addition to or in replacement of the way MT usually does it. My suggestion is that you do this temporarily and in parallel with normal MT daily archiving. This way when you import your old Radio entries using my Radio Exporter.root tool you'll get archives created in the same path locations as those made by Radio. Once the import is done you could remove this template. This would leave the rendered files in place, just be sure not to delete them in the future.

There are other ways to do this. One is to use the mod_rewrite module available in Apache. Since many sites don't run this module it may not be a viable option. There's also the .htaccess directives for Redirect and RedirectMatch. You could capture the incoming old links and redirect them to the new location. If your site allows use of these directives in an .htaccess file this is probably worth considering. Use of .htaccess directives introduces other overheard factors to it's not without it's issues.

I'll stress this point again, these steps won't work if you're hosting your site on the radio.weblogs.com server. UserLand doesn't support your use of functions like .htaccess and redirects. Considering the reason you're using redirects it's not terribly likely they're going to start either. One one level it would actually help lighten their load. Allowing departing users to configure a redirect would avoid their servers forever shovelling out your old data. But that'd be smart and, well, that's not been a dominant feature of hosting services offered by them.

So, if you're turning off the Radio and migrating to MovableType you can take steps to ease the transition. Give it a try.