Move a Reseller user to Shared user
You can expel a user in your reseller plan and create an independent Shared hosting plan for his service. In order to create a new shared hosting plan with your user domain, at first you are supposed to create the desired shared hosting plan with another domain name (note that two different plans with one domain name is impossible in DNP control panel) which doesn't exist in dnp cp.
After creating the new plan you should make backup of all data of the existing domain in last hosting service and then when you were sure, delete this domain and next add this domain to new hosting plan.