I've been trying to find a way to do this for quite a minute. In all my conversations with Workfront ninjas, consultants, and WF support I've always been told there isn't a way.
What I've done instead is hide the standard feilds from the form and replace them with custom fields. Then I use Fusion to push the custom field answers into the standard fields.
For example, instead of asking for a planned completion date, we want to ask for a go-to press date. So we don't ask for a planned completion date and push the go-to press date answer into the planned completion date field.
Make sense?