That NAT must be done on the client side, as others have stated, but since you are using OpenVPN there is a chance you can pull it off.
I have not tried this but OpenVPN also has built-in NAT:
--client-nat snat|dnat network netmask alias
This pushable client option sets up a stateless one-to-one NAT rule on packet addresses (not ports), and is useful in cases where routes or ifconfig settings pushed to the client would create an IP numbering conflict.
network/netmask (for example 192.168.0.0/255.255.0.0) defines the local view of a resource from the client perspective, while alias/netmask (for example 10.64.0.0/255.255.0.0) defines the remote view from the server perspective.
Use snat (source NAT) for resources owned by the client and dnat (destination NAT) for remote resources.
So you could try this in the client config:
client-nat dnat 10.100.0.0/255.255.255.0 192.168.0.0/255.255.255.0
That could be pushed in a client-specific override as well.