Odd relay request for mail

Hello everyone… I have what is going to sound like a strange request for a temporary configuration while migrating my email server to Nethserver.

Currently I have my working server at a remote location with realtime sync to my new site.

Nethserver mail is configured and working with SoGo and great so far

configured imapsync for users to manually migrate mail from the sync location, to nethserver - working great

I can individually redirect email for a user from the old server to nethserver - works great

Smarthost is configured to use my external relay system - works great.

current mail flow is:
source -> old mailserver (redirected to NS) -> delivered to local NS user

As i am letting people choose when they migrate, all they have to do is send an email to an executable account on the old server, which reconfigures it to redirect any further incoming mails to the NS server - works great

they then use the imapsync interface and migrate everything from the sync copy to NS - works great.

My issue, is now I have two mailservers collecting mail for one domain. on the NS server, i would like to force all outbound mail, even if it is for a local user, to use the smarthost as the recipient may not have migrated to NS yet, but still receive email when it is received.

This is a temporary configuration and once all users are moved all I need to do is update my DNS to point to the nethserver and remove whatever forces mail for local users to be sent to the smarthost

Is this possible? everything but sending all outbound mail to the smarthost works great so far.

Thanks !!