VPN

I have VPN set up and on the m0n0wall side connected to one local subnet. I would like to connect it also to a second local subnet but the webgui does not permit to configure multiple local subnets. Suggestions or upgrade possibilities?

You can set up one IPsec connection for each subnet.
See this FAQ : http://doc.m0n0.ch/handbook/faq-ipsec-multiple-subnets.html

Also make sure that if the additional subnet is not local to you m0n0wall, you set up the appropriate static routes so that traffic between it and the far-end passes.