I need to create a post-function to assign the issue to the current user only when the user is not member of a particular project role.
Is it possible?
You have to use Set a field as a function of other fields post-function with the following configuration:
Setting rule used is:
[!isInRole(%{00020}, "name_of_project_role")]%{00020}
Note that:
Once configured, transition will look like this: