Ringtones - Why Can't I.....?

We just added our daughter to our Alltel service with a Samsung SCH-R500 Hue
phone, and she dragged her fossil-aged father into the age of wallpapers and
ringtones. Silly me, I thought cell phones were for making and receiving
phone calls..... Anyway, we've figured out how to download a ringtone from
Alltel, and how to actually create and use custom wallpaper, using an
external memory card. I've got some pretty good audio s/w on my computer, so
I don't understand why we shouldn't be able to create our own ringtones from
audio files, then use the same memory card to install them on the phone. I
cannot find any info regarding this procedure, or any info regarding
ringtone file formats, size restrictions, etc. I found some software that
claims to create custom ringtones, but this stuff still seems to require
phone #'s to download it directly to the phone. Why can't I create ringtones
as a stand-alone process and store them to the memory card? Any advice and
feedback is appreciated. Thanks,
JW