This type of modification is beyond the scope of Customizer and would need to be a source code change to accomplish to ensure data integrity.  I recommend contacting your reseller and see if they can do this for you if you do not own the Sage 500 SDK and have the source code for GL.  If your reseller cannot assist you with this, they can contact us (440-352-4700) to build this customization for you. 

If you would like to know more about E2bTek, visit our website at http://e2btek.com/ 

There are a few other options for getting source data like that into Sage 500. Perhaps you're already aware, but if not, here's some alternatives:

You may be able to use DataPorter, on the Enter Journal Transactions screen. Once you map your data to an excel file that matches the layout of the journal entry DataPorter controls mapped, you could copy the data in via DataPorter (Alt-F8 on the EJT screen).

You could use the GL Import utilities: General Ledger Module \ GL Utilities \ GL Import 

Choose the Transactions "Data Type" and layout your data for this mapping.

Or, use Data Import Manager Module to map and import your data.

Darrick