Assuming that you don't want to negotiate an additional tunnel between Site A and Site C directly, you can do this simply by negotiating the appropriate IPsec policies on the two existing connections (and, of course, allowing forwarding of that traffic on Site B).
Between Site A and B you negotiate 10.0.0.0/8
on Site B and between Site B and Site C you negotiate 192.168.10.0/24
on Site B. This enables Site A and Site C to send traffic to those additional subnets to Site B, which then also can forward that traffic appropriately (it needs forward policies that allow traffic from 192.168.10.0/24
to 10.0.0.0/8
and vice-versa).
If you use IKEv2 you can simply add the additional subnets to Site B's local traffic selector for each of the two connections. And similarly on Site A and Site C for their remote traffic selectors, or you may configure 0.0.0.0/0
there to let Site B do the narrowing to the two subnets.