As far as I know, there's no way to transfer the built-in MIDI ringtones from a 7290 to an 8800.
Even if you could transfer the files, they wouldn't sound the same. The info embedded in a MIDI file isn't the sound. It's a series of commands needed to recreate the sound. The beep-like quality of 7290 ringtones comes from the limitation of the audio chip. The same ringtone when played on a more recent BlackBerry might sound like a real phone ringing rather than a series of electronic tones.
You could try the ringtones at
http://davidenglish.com/wml/index.wml. Visit the site with your BlackBerry and choose "Non-Musical MIDI Ringtones." The second group has 7290-like ringtones. The third group has some very good vintage phone ringtones. You can also access the vintage ringtones at
beepzoid » Vintage Phone Ringtones.
DavidE
davidenglish[.]com
beepzoid[.]com