There's no law that says you cannot wear a ring on your wedding finger if you're not married.
I used to wear a ring I liked on my wedding finger when I wasn't married because I preferred to wear the ring on my wedding finger instead of the other fingers.
Most people wear the rings on other fingers instead of on the wedding finger which is their belief and that's okay too but there's nothing wrong with wearing any other rings on your wedding finger if you want too even if you're not married.
You can wear any type of ring on any one of your fingers that you want too as there's really no rules or laws that say otherwise.
If you prefer to wear a ring on your wedding finger when not married or engaged you certainly can and should.