Can the Jira default workflow be amended to include more steps and transitions. How do you deactivate the jira default workflow ?

Why am I asking:
We have many projects set-up on Jira. If any project needs to change we need to deactivate the workflow by selecting workflow scheme = none.

This will then ask to "associate". It tries to map the steps in the project to the default jira workflow.
Our projects might have 15 steps. The jira default workflow has 5-6 steps.
This means we will loose our steps in the project if they are not one of the steps in the jira default workflow.

Please see attached the Steps we have set-up on one of our projects.

Regards
Colm