You could try a Snom M3 Repeater. This will extend the range. If you can arrange the base station in the middle, and repeaters around in a circle then you can cover a huge area. Calls hand over between repeaters as you walk.
Other option is something like a Gigaset S685. The handsets have a `best base` feature where you can register a handset to more than one base station. You need to be a little bit clever with setting up SIP accounts. Each handset needs a SIP account for each base. These then have to be setup to all ring together. This solution does not allow handover while calls are in progress.
A long term option would be the Snom M9 phone. A roaming feature is in the roadmap for this phone, but probably at least 1 year away from launch.