Been playing with Auto Index.

So here’s my situation. We have an Accounting File Cabinet and a Checks File Cabinet. What I am trying to do is use Auto Index to place the Check Number from the doc in the check cabinet into the invoice of the doc in the Accounting cabinet. A one for one exchange works fine. The problem is when the check doc is used for more than one Invoice. I realize that the Type in the Database Fields needs to be the same (text for text or numeric for numeric). Is there a way to enter multiple Invoice numbers into one Database Field so Auto Index can put them into the proper Invoice?  I tried making the Invoice Field in the Checks Cabinet a Keyword Type, but that doesn’t show up when trying to set up the workflow. Any suggestions would be appreciated. I’ve tried playing with the SQL statement when using the Database connection, but I don’t know sql and can’t get a proper statement.