Extend the webform submission
You can't create a new value to non-existing field, since the data has no place to save to the database. Ideally you should create a hidden field for your data and modify it in your handler on presave event or use hook_ENTITY_TYPE_presave. If you have multiple values to add, serialize it or save into JSON and assign to the hidden value.

You can also use Computed token field where you can define and place your custom token (with any dynamic data) into value of that computed field which can fetch and store the data dynamically.

Related: Is there a webform way to store additional data on a submit handler?