Christian,
What you are describing sounds like "overlapping networks". Cisco IOS routers, VPN Concentrators, or PIX Firewalls can be used to connect overlapping networks. If you search Cisco's website for "overlapping networks" there are configuration examples for the above-mentioned equipment; which one you use depends on what equipment you have, and how it's connected.
From IP Addressing Services, NAT Frequently Asked Questions:
"IP address overlapping refers to the situation where two locations that want to inter-connect are both using the same IP address scheme. This is not an unusual occurrence, and will often happen when companies merge or are acquired. Without special support, the two locations will not be able to connect and establish sessions. The overlapped IP addresses can be public addresses assigned to other companies, private addresses assigned to other companies already, or from the range of private addresses as defined in RFC 1918 . Private IP addresses are un-routable and require NAT translations to allow for connections to the outside world.
The solution involves intercepting DNS name query responses from the outside to the inside, setting up a translation for the outside address, and fixing up the DNS response before forwarding it onto the inside host. A DNS server is required to be involved on both sides of the NAT device, to resolve users wanting to connect between both networks.
NAT is able to inspect and perform address translation on the contents of DNS A and PTR records.
For more information, refer to Using NAT in Overlapping Networks."
Here's the link to the above reference:
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_configuration_example09186a0080093f30.shtml
Hope this helps.
