Using a file explorer, go to the Oxygen XML Editor plugin frameworks directory. Select the sdf directory and make an archive from it. Move it to another Oxygen XML Editor plugin installation (eventually on another computer). Extract it in the frameworks directory. Start Oxygen XML Editor plugin and test the association as explained above.
If you create multiple document type associations and you have a complex directory structure it might be easy from the deployment point of view to use an Oxygen XML Editor plugin All Platforms distribution. Add your framework files to it, repackage it and send it to the content authors.
When deploying your customized sdf directory please make sure that your sdf directory contains the sdf.framework file (that is the file defined as External Storage in Document Type Association dialog shall always be stored inside the sdf directory). If your external storage points somewhere else Oxygen XML Editor plugin will not be able to update the Document Type Association options automatically on the deployed computers.