The Universal Ringtone Converter is free and converts standard Nokia Composer ringtones to various brands of mobile phone ringtone format. There is no download required, just use it from this page.
 

3 Easy Steps to convert your ringtone:
Step #1: Copy & Paste a Nokia Composer ringtone into the input format box.
Step #2: Select a output ringtone format from the output format option.
Step #3: Click "Convert Ringtone" to convert ringtone.