OpenVPMS doesn't directly support data migration from other veterinary practice systems.
It does however provide a plugin for Pentaho Data Integration (PDI) that can be used to get data into OpenVPMS.
The plugin works with PDI 9. This can be obtained from here.
Extract this zip file to a directory. This will be referred to as <PDI_HOME>.
To install the OpenVPMSLoader plugin:
See also Implementors Forum|Data Migration