If you have an existing Office file, you can still save it to Aras, but the options will be slightly different than creating it in Aras to begin with. To save an existing file to Aras, just open it in the Office application, and pick Save to Aras.
Behaviors compared to creating a new document are:
Only those Document Types not requiring a template will be shown, since it is not possible to use a template for an existing file
Not selectable, since the file already exists
You can select any item to link to, based on valid item types for your selected Document Type
All capabilities are the same
The File Name field will not appear regardless of Document Type. If the Document Type does not automatically rename the file, it will just use the existing file name.
After saving to Aras, you will see the Aras ribbon updated with properties as shown below. Note that in this case the Document Number and file name are created based on an automatic sequence number - D100008.