It can be downloaded here at Github.
The tool basically is designed to support any social platform, but the first release is only capable of supporting oxwall's version 1.7 and 1.7.1.
The import source is a CSV file in an arbitrary format, including avatar pictures, custom question fields, group and role memberships. The mapping of the source fields to the target database fields is fully customizable.
Each new imported user receives a free customizable email.
The tool is written in java and needs a Java8 runtime on the executing machine. If your oxwall database allows remote access, the tool can be run on any external machine which provides a Java8 runtime. Otherwise you have to run it on your oxwall installation machine.