Show field from SI on form 20 GL Entries

using NAV 2009 SP1, is it possible to display a field from the posted Sales invoice on form 20 GL entries ? I have a serial number field on my sales invoices that am needing to see what transactions in the GL account relates back to the individual sale

You need to take variable and put the value of serial no as comma separated and display .

There is no easy way.
Perhaps a new calculated lookup-field in table 17 could be the solution.

But I would do it like this:
In form 20 (or more likely - outside standard objects) I would create a function that
-1 determines whether an invoice is the base for the current record (GlEntry)
-2 tries to find the invoice
-3 tries to locate the line that “contains” the serial number
-4 fetches the serial number
-5 returns the required data

Define a textbox on the form; set the function as SourceExpression.
Beware - this may cause your NAV to work slow when scrolling ledger entries in form 20.