Is it possible to force a branch office to go to the HQ (through a VPN) for a specific external traffic range. For exemple if I want the range : 123.345.567.0/24 going through the VPN to the wanos in the HQ.
Yes, that would one possible way. Also you would need the BranchNetwork to either use BranchWanos as gateway or add a route on the BranchNetwork workstations: