Sandboxie-Plus does not contain a proxy itself, but recent versions got a feature to force sandboxed applications to use a predefined Socks5 proxy.
Those proxies can be different for different applications and for different sandboxes.
Sandbox Options > Network Options > Internet Proxy
The first column allows specifying a program, otherwise it applies to all of them by default. If you have a proxy or proxies listening on certain IPs and ports, you can enter them and redirect traffic through those proxies. The proxies have to use the desired IP addresses.