You will need to use .CSV file to import the Secondary-Proxy-Address. Do this, in Exchange Admin, use Directory-Export (Mailbox Only) to export the contents of your GAL. Next, modify the .CSV which was created for you...Delete all columns except: Obj-Class, Directory Name, Alias, Home-Server. Insert new 2ND Column with header of Mode. At end, add column Secondary-Proxy-Address. Now, for all the mailboxes you want to update, insert the approriate address into the Secondar-Proxy-Address column. In the Mode column for each mailbox, insert Modify. Save file (you can/should delete all mailboxes from file that will not be updated).
Now, back in Exchange Adm Pgm, Directory Import (only servers in current site, modify .CSV file for all sites), make sure MULTIVALUED PROPERTIES is set to Append.
Hope this helps.