In a paternity suit decree naming the father and mother Joint Managing Conservators, can the mother open a UTMA and be shown as the custodian and then appoint the step-father as the successor custodian?

That seems like an inappropriate move. The decree apparently requires the two parents to act jointly as conservators of the child and his/her assets. Putting funds into an UTMA with only one custodian runs counter to the decree.

Any account with the child's funds should require both parents' signatures for all withdrawals and list them as joint conservators.

The courts sometimes don't really consider how cumbersome the implementation of their orders can be!