Actually, if you have registry access you can. Currently I have Bender from futurama saying to kiss his shiny metal , as my text tone.
sync through zune, some tones you like, mp3, under 40 seconds, genre: ringtones. Then go to your wp7root tools explorer function. Copy/paste your tones from My documents/my ringtones into the Windows Folder, then go to your registry local machine......dang, I forget the path between local machine and OEMNotifications, but you search custom alerts on xda if you can't find the notifications from local. Amywho, in oemnotifications, create a reg value, name it EXACTLY what your tone was called that you put in the windows folder. Save. Then just go to your sounds page in your settings and your tone will be available for SMS, email, voicemail, etc. Sorry for the incomplete instructions, and its more of a hassle that a lot of people think its worth, but check xda to fill in my blanks if you want to do it. Enjoy.