You mean you want to override the user object we provide to have some custom fields? If that's what you mean, then no, there isn't a way of doing that.
You can:
1) Store the custom fields against the supertokens_id in your own db and fetch that whenever needed.
2) As you mentioned, store those custom fields in the JWT payload when a session is created and extract them from the session when needed.
I would go with method (1) if the custom fields are retrieved rarely, whereas go with option (2) if those fields should be accessible from the frontend
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need to be accessed very frequently.