you'll have to use source NAT for each of the 192.168.10.1 (NAT it as coming from a different IP) on their firewalls in the NAT policies menu(or at least one of them it depends if the 192.168.10.0 subnet clashes with any other destination networks)then add the Natted IP addresses to the local networks on VPN and and Head office add the Natted IP's in the Destination networks,then for example from the Head office when you want to go to 192.168.10.1 at Site a you go to 192.168.20.1, and for site B you would go to 192.168.30.1, obviously change to use IP subnets you aren't using this is just an example
Answers
you'll have to use source NAT for each of the 192.168.10.1 (NAT it as coming from a different IP) on their firewalls in the NAT policies menu(or at least one of them it depends if the 192.168.10.0 subnet clashes with any other destination networks)then add the Natted IP addresses to the local networks on VPN and and Head office add the Natted IP's in the Destination networks,then for example from the Head office when you want to go to 192.168.10.1 at Site a you go to 192.168.20.1, and for site B you would go to 192.168.30.1, obviously change to use IP subnets you aren't using this is just an example
no Natting needed on the Head office firewall