Earlier I already patched the DHCP-server to make it do as I wanted: give all local pc's an second dns-server. Now I've found a nicer solution: the dhcp-panel at this page.

SME-server's dns-server is a bit slow sometimes, there are websites I can't reach such as parts of www.nu.nl (Dutch news). As the second dns-server I added my ISP's dns-server.