Use Screen4 or Machscreen to add a button to the screen.
Yes, what OEM Code of Button should it be?
An XML file is just a text file so you could do what some folks do and just hack it. Alternatively you could do it right and use the XML Document Object Model (DOM). Depends on which VB you're using so search tinterweb for xml document object model in vb.
If we are using Mach3's Cypress VB for accessing XML, is it possible? I have not heard of a tutorial of read/write XML using VB6. T_T