Add Merge Fields to a Word Document before adding a DataSource

Adding merge fields to a Word document without adding a datasource to it – and anyone working with multiple datasources, or programmatic mailmerge setup wants to do this sometime – is not at all obvious when the mail merge button is greyed out..

You can do it, unobviously via Insert->QuickParts->Field:

Choose MergeField from the list on the left, and then you can type in your merge field name.