I am not very clear on what you are wanting.
Are you saying you are setting up a VPN to another site that has overlapping IP's?
If that is the case then you or the other end will have to NAT to an IP block you agree upon or force them to NAT to their public ip since all public space is unique to each site.
One thing I have done is for cases where the client would not use public, I had 2 IP ranges set up specifically for client NAT. I would assign them a pool of IP's just like an ISP and have them NAT to the IP block assigned.
For example:
Client A Has 10.50.0.0/16
This IP blockis being used on your network already.
I would assign them a block 10.199.0.0/29 or whatever size they needed, and they would need to set up NAT. Then when building the ACL for traffic to that client you use the block assigned.