In Version 2.30.0 of SNIP it’s possible to also add user meta data to your schemas. Here is how you it works.
Fields
You may ask what kind of data you’re able to add. Here is a list of meta fields that WordPress adds out-of-the-box. However you can always enter a field name from a third-party plugin. All you need is the name of field.
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If you want to access fields from the wp_user database table instead, use the according field types.
Complex fields
If a field has the form of an array or object, you can maybe also use it via SNIP. For this follow the guide here.