So, I assume the field the user is entering has the value list displayed. If the item they want to enter is not in that list displayed, you want them to be able to add that item to the Master list. If that assumption is correct, the "down and dirty" way is to have an "add" button close to where they are entering the item. They would be instructed to click the Add button when their item doesn't display in the drop down list. The Add script would allow them the enter a new value in an Orders::global field. When they click OK, the script (with full administrator privleges) would go to the Materials layout and add that value (as a parameter), then return to their layout, clear the global field and place them in the item entry field so they can choose the new item from the drop down list. I hope that is something close to what you want to do.
Thanks Mark, I'll give it a try!