We have a Windows Server 2008 R2 with a DNS, but the problem is our website is available form a different IP form outside (say 83.3.3.3) and different from inside (say 192.168.0.3). So is it possible to add a rule that (localy) would resolve www.mydomain.com to a 192.168.0.3 (while still resolving to 83.3.3.3 for the outside world)? Note that I would like to set it for all users of the internal network - something like etc/hosts but for all internal users.
If I'm undesrstanding what you're asking, you'll need to create a zone on the internal DNS server for mydomain.com and then add an A record for www that resolves to the internal ip address of the web server.
You can use ISC Bind, which supports views. It return different addresses to different hosts.
http://zytrax.com/books/dns/ch7/view.html