This plugin wasn’t built to bring in existing records that were created before it was installed. Because of that, it doesn’t do bulk imports well unless you’re comfortable writing JSON code. Here’s one way you could try to do it (I would recommend not doing this in a production environment first):
1. Set up the plugin and make sure that an import of at least one new member from Salesforce into WordPress works.
2. Do an export from the plugin of the object map so you can see how the JSON structure works.
3. See if you can expand that JSON structure to include all of the existing members with their data.
4. Do an import of the expanded JSON into the plugin.
If all of those steps worked, you’d be able to update the records in Salesforce and have the changes come into WordPress.
To be clear this is a lot of work, and it might not work with your WordPress data structure depending on what plugins you use for member data. But that’s the only way to do it, currently.