Don’t know if I completely understood your question, but here it goes:
If you want to migrate Notes to Exchange you’ll need Quest NME;
If you want to maintain both realities, you’ll need something like Quest Coexistence.
You can’t live and integrate both without paid software (like above).
For free, the most you can do e create on both servers a internal domain/policy like something.local.
Next you need to setup an aditional connector in exchange with a smart host to forward all email sent to the something.local to the IP of Lotus server.
(For this to work all internal (*@something.local) email accounts for this relay must exist and need to match up:
In the Exchange server the address firstname.lastname@example.org must be equal to the Lotus server email@example.com
Hope this Helps,