I can give you some pointers that might help you but you need to understand that THIS IS THE *WRONG* WAY TO ACCOMPLISH YOUR GOAL! You need to renumber. You will run into issue without renumbering. You will not be able to share DNS or most likely have computers on the same domain across a NAT'd network like this.
Is the site with the servers (the home office) on the same IP adddress space as the other branch offices? Do the branch offices need to be able to talk to each other or just to the home office.
Essentially what you would do at each remote site is setup a NAT that would take each IP in the local subnet (lets call it 192.168.0.0/25 and translate it to an IP on a different subnet 10.10.1.0/24. At the main site your VPN would be setup with the 10.10.1.0/24 as the remote network. For the next sites you would use a different subnet.
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