we have done similer kins of thing but for SAML integration.
once you get the data back from post get the role info and account info,fetch the account and role object from DB and user them
some time role name might not be same as exactly as u get in rest reponse you u might want to maintain a mapping in the App for them in some entity.
You can make different decisions in a microflow, one per role. Based on the decision, you can retrieve the correct UserRole from your database, which you then link to the correct user.
You cannot create new role dynamically in run time.
Mendix User roles, related module roles and access is 100% managed in the model.
There are ways to setup the security dynamically, but therefore we need more information about the use case in your situation.