You are probably going to want to do a route based dial up VPN. Think of this as a hub and spoke VPN, with the dial up client being one of the spokes. Also, you are going to want to configure an IP address pool, so that you can route the traffic back from site B to site A, then to the dial up VPN.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB15272 talks about route based dial up VPNs.