There is an article titled How to make JIRA applications fields required in a workflow transition that may be of assistance:
A particular field should be required to perform a transition. However, JIRA applications only allow a field to be set as Required in the global context.
Use the Fields Required Validator shipped with the JIRA applications Suite Utilities add-on to make the field required for a transition to be completed.
Hopefully that gets you where you want to go!