Do you still need help with this?
Yes, it is possible to add extra fields to the shipping form. Goto Virtuemart admin area. Click "Admin"->"Manage User Fields" and see the list of current fields.
You can click [New] and create any custom field you want. To make the new fields show up in the shipping form, just select "Show in shipping form".
If you need more help, or want me to do it for you, please let me know.