If you'd like to add QBO to your QBO Payroll subscription, then you can do it by navigating to your Billing & Subscription page to upgrade your subscription. Here's how:
Click on the Gear icon⚙ and then select to your Account and settings.
Go to the Billing & Subscription tab.
Select Upgrade your plan in the QuickBooks Online area.
See the available plans and then hit Choose plan.
Go through the on-screen steps to upgrade your plan.
Feel free to refer to these articles to know more about handling your subscriptions: