Hello, most devices don't have an option for the said function. In this case, I don't have any option on my Nexus 5 either.
I have a music player installed called "NexMusic" its simple, fast, feature full. And the best thing is that you can set custom mp3 ringtones just by touching and holding down on the desired music file.
Hopefully this may be a way around the ringtone option.
Simply search "NexMusic" on the PlayStore, its free.
iOS7 Kik Messenger | Svelte UI CM11 Theme | Active Launcher