Hey jayasri,

User roles are decided by the grantable roles in the run time, so you would only see the roles that you are eligible to create from the user roles setting,

For this implementation, there is a workaround, 

Just create a NPE with attribute (say RoleName) and then associate it with your entity, now using a microflow, retrieve all the user roles from the database and the create an NPE object for each user role and then,

as soon as the user selects a user role From the list of NPEs, 

you can then implement what you want to do using a microflow and assigning it to the particular user role,

Hope it helps!