Is there a way to configure VPN with Many-to-Many Source NAT?
Yes like Policy-Based, Route-Based and GRE over IPsec Site-to-Site VPNs are compatible with Many-to-Many NAT. They are used to translate network to different IP address ranges over the VPN.
Is there documentation you can provide on how to configure this one a Digi IX20?
Digi IX20 Local LAN <–> NATed LAN <–VPN–> Remote LAN