A. Not easy: Redirecting…
sys-net ---- sys-firewall ------ not-whonix
| ^
| | QubesRPC
| v
anon-sys ---------- anon-whonix
B. Not easy & more dangerous: have both anon-whonix & not-whonix VM connect to an intermediate proxyVM and use routing / iptables rules to selectively send traffic to destinations
sys-net ---- sys-firewall -----| |----- not-whonix
| | |
| routerVM
| | |
anon-sys --------| |----- anon-whonix
You might get more help from qubes-users mailing list. Tip: Generalize the question: “How to set up networking between 2 VMs using Qubes RPC?”
Prior discussion: Allow Networking between Qubes and Whonix Workstation