For the parent:

Accounting>Chart of accounts>New

use the same Account type and Detail type as the existing accounts (that will soon be sub-accounts!)

What makes it a parent account is that the box "is sub-acct' is NOT checked.

Save and close.

Now, edit the existing accounts so that they are sub-accounts of the parent:

Click on box to left, and then all the way to the right for Action, choose Edit.

Check box "is sub-acct" and use dropdown to choose parent account.

Save and close.

View the Chart of accounts to verify.

Sub accounts should appear indented under the parent.