You cannot individually add sub-accounts. You add the Major/Rollup account, and it will automatically add the sub-accounts
If you added new sub-accounts to a roll-up account in the accounting system, and you want those changes to reflect in the budget system, go to the section in the budget system that needs to be updated and click the refresh icon.
You can also remove sub-accounts you do not want. Right click on the sub-account number you do not want in your document, and click “Remove This Account”