The default AWS CloudFormation step can be found here, but as you can see it is not exposed as a script. This means that you will not be able to manipulate the step using scripting.
In regards to community steps and accounts, we have some known issues around binding the accounts here and here. You should be able to provide the access key and secret key as part of the parameters using the following bindings:
I managed to get this to work with a custom library template, however did not have much success using the CloudFormation community template as is. I have therefore included a modified version of the community template which allows you to bind the AWS account direct. You can import the contents of this file by navigating to Library -> Step Templates and clicking on the Import button.
cf-custom.json (13.0 KB)
Hope that helps!