Normally, Sandboxie can block network access, but does not have an option to use a different IP.
I guess that it might work with the SOCKS5 proxy, where you can set each sandbox or each program to connect to a different SOCKS5 proxy that has been set-up in advance. This feature is not available unless you have a certificate that permits you to use it. I am not aware if it can be tested, but in case of licensing questions, please get in touch via email, David can surely help you out.
The feature can be enabled in the Sandbox settings > Network Options > [Internet Proxy]