[Eurisko]

The file type actually remains the same. iTunes just looks for the extension name. In iTunes, first go to preferences -> advanced -> importing and select "Import using AAC.

Then right-click on the file you want to use as a ringtune, and you'll see "Convert to AAC". Once this is done you'll have two files under the same name, iTunes doesn't delete the original.

Again, right click on the new AAC file that's just been created and select "Show in Finder". Move the file with the extension .M4A to the Desktop. On the desktop, rename that file to .M4R, then double click on it. It will be placed back in iTunes as a ringtune.