Can we create new custom user roles in Smartsheet?

Does Smartsheet provide any facility to create custom roles and assign it to user using any REST API? Currently we are only able to assign Group Admin, System Admin or Resource viewer roles using /User endpoint. I also observe some roles such as owner, admin, editor, viewer or commenter. Does it also related to Smartsheet user? If yes, How can we assign it to user using REST API?

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