Yes, you can make the existing customer a job in QuickBooks Desktop, DanBu.
To perform this process, you need to do it from the Add/Edit Multiple Customer window. I'm glad to walk you through the steps.
- Open the Customer Center.
- Locate and right-click the customer name you want to change.
- Choose Add/Edit Multiple Customer:Jobs.
- Go to the Customize Columns button.

- From the Available Columns section, select Job of: and click Add.

- Place the Job of: item below the Name by pressing Move Up continuously, then click Ok.
- Look for the customer you want to alter and pick the new Parent customer to connect it to in the Job Of: column.

- Click Save Changes.
Please note if that customer already has transactions, they will now be assigned to the new parent customer (as a part of the job).
To help you organize the cash flow and track sales, receivables, and profitability more accurately, check out this guide: Get started with customer transaction workflows in QuickBooks Desktop.
If you're using our payroll service, I'm also adding this article to help you get ready when it's time to pay taxes: Year-end checklist for QuickBooks Desktop Payroll.
If you have other questions in mind, feel free to reply below. I'll be delighted to answer them for you.