Can you elaborate on exactly what you are trying to accomplish ?
I think what you are asking is that if you have a drop down list (combobox) and you store an ID in the document, in the transformation you would like to get more of the information from the row of the stored id in the custom table.
There are several approaches, I traditionally just modify the query or view and join the tables on the ID, that way the additional columns from the custom table are in the underlying data. Although if you are not using unique ID's in your custom table and storing those values in your document, then you will have an issue.
Please explain in detail what you are trying to accomplish.