Typically, VPN-Clients get a reserved pool of addresses that doesn't overlap with anything else. But it's perfectly fine to assign addresses out of the internal network of the ASA. That even makes routing easier as the internal infrastructure will automatically route client-traffic to the ASA (if there are internal routing-devices).
All you have to do is configure an IP-pool with addresses out of the internal network and assign that pool to your tunnel-group or group-policy and don't forget to NAT-exempt that traffic.
Make sure you document well that these addresses are reserved now and shouldn't be handed out to internal clients.
