If it's not made to convert to a toddler bed, it's really not safe to try anyway. For your child's safety, buy a toddler bed. If money is the problem, go to Once Upon A Child. They are not as expensive as a store, and their equipment is in very good condition.
It is very simple to convert a crib to a toddlers bed. You just need to remove the side railings by folding them under the bed. It would be automatically converted to a bed that a toddler can use.