Add a field to a form that is not in the datasource

In general, How do I go about adding a field to a form if it is not an available field in the form's data source?

Specifically, I need to add the field Document date to the form VendShowspecification (in the A/P payment journal > Inquiries > Marked for Settlement)

The field DocumentDate does not exist in either of the two Data Sources available in this form:

SpecTrans or TmpcustVendTrans

Any help would be appreciated!

Dan