Can I add DNS but keep those provided by DHCP in OSX?
Solution 1:
Since Mac OS X 10.6.3 you computer keeps a list of all DNS servers it has ever seen and automatically picks the fastest one for you. So you can simply add the new one and be happy with it, as your Mac will remember the ones provided by DHCP. If you want to define a strict order after which Mac OS does it´s lookups, best follow this how-to.
Additionally, to see the current DNS search order (and search domains), run: scutil --dns from the Terminal. Or, to quickly check which DNS server you are actually currently using, run for example nslookup apple.com.