According to this review :
http://www.mobile-review.com/review/...-k750-en.shtml You can make custom ringtones using MusicDJ which is installed on the phone.
However as the phone is capable playing MP3 files as ring tones it may be simpler to create the tone on a PC, or record it on a PC thne transfer it to the phone by blue tooth, cable email etc.
Have fun.
By the way, if you listen to the default SMS tones on old Nokia phones I think it actually plays SMS in mores.