InfoPath Dev

Per the attached image, I am looking to see if its possible to change the Saleperson (Header) field into a drop down field box?

Hi - yes, you can. I generally click on the form where I want the control, then right click the field in the field picker to get the options shown in the attached.

How do I add a new secondary field?

This depends on the data source for the secondary connection. If it is an XML file, you can go to Data > Resources and export the XML. Add any new fields, import, then walk through the data connection wizard again for the file to include them in the schema in the form.