If you are not already having the StrongVPN link out of siteA as the default gateway for pfSenseA, then you will need to use policy-routing on the OpenVPN rules mentioned by @Reiner30 - in the Advanced of the rules, select the StrongVPN as the gateway for the traffic.
Make sure your pass policy-routing rules on incoming OpenVPN specify something like "source <network a|network="" c="">destination (or destination )" - you don't want to route packets from network a to network b, straight past and out the StrongVPN.</network>
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