The lazy solution: switch between source types
In the source manager, when you change the type of a source, different fields will become available. You can switch to a type of sources which has the fields you want, and fill them in. Then, when you switch back to the type you want, the data will be kept
although it looks like the fields are gone.
So you can switch your source type to "Patent", then you can enter "Thesis" in the "Type" field. Afterwards, just switch back to "Report" and fill in the rest of your fields.
The good solution: edit bibform.xml
The above works if you only have one or two entries. If you have dozens of entries, it becomes a stupid thing to do.
The order and availability of the elements for a given source type in the source manager is defined in a file called
bibform.xml. Normally, this file can be found in
C:\Program Files\Microsoft Office\Office12\1033\Bibliography
where the 1033 could be a different number for you. It represents the localization of your copy of Word (1033 = US English).
If you open the file you will notice a huge bunch of xml code. Its format is pretty simple and straightforward. For each type, there is an entry in the form:
<Source type="Report" display="Report">
<!-- all definitions -->
The children of a source element are the different fields that will be available in the source manager for that type. For example:
<Sample>Example: Adventure Works Monthly</Sample>
What you can do is copy paste the type field from another source type inside the Source element for Report. You would want to add the following code inside the Source element:
If you save the file and try to edit a Report source, the type field should now be available. Note that you might need to select "Show all fields" depending on how you defined your important fields.
BibType is a way of automating the above process to work on all languages. For small changes like yours, it's easier to do just do it by hand.