Altering OAuth flow

[craigrbarnes]

I am working with an OAuth identity provider which has a different "flow" than what is shown in the docs.
The step in the diagram labeled "computes signing options ..." is actually done by a call to our identity provider (idp). It computes the URL to the service login page. I would like to know if there is a way to change the flow in the yellow circle below? Specifically can replace the compute signing options with a call to get the correct URL + all parameters and pass that to the "GET github.com/login/oauth/authorize" step?.

A second question is: can I use a computer CSRF string?