If you can first dump all your users (for example, by writing a script that calls your API in a loop) and save them in some easily parse-able format (like JSON or CSV file), a few more options come to mind in addition to what Michael has mentioned:
1. You could try to import CSV/XLS file from oxTrust web UI as described [here](https://gluu.org/docs/gluu-server/4.1/user-management/local-user-management/#import-people-in-oxtrust). It may happen to be quite limiting though, and was reported to handle huge blobs of users not so good
2. In 4.1 and later oxTrust now offers REST API that helps automating many UI tasks. In particular, [this function](https://gluu.org/docs/gluu-server/4.1/api-guide/oxtrust-api/#createperson) allows you to create a user. So you could iterate through your dumped user data by calling it, pushing your users one by one
3. You could employ [SCIM REST API](https://gluu.org/docs/gluu-server/4.1/api-guide/scim-api/)
But as Michael has pointed out, to get timely assistance with any of these approaches, you probably would want to have a support contract.