Yes, in most cases you can sell your own directly contracted hotels alongside the ones that come through the hotel booking API, although whether this is switched on for your account depends on the plan you are on, so please confirm it before you promise anything to a property.
The way it normally works is that you load the hotel yourself. You add the name, the address, the photos, the room types, the board basis and the rates you have agreed with the owner. You also set how many rooms you hold on each date and the cancellation rules you want your customer to see. Once that is saved, your own hotel appears in the same search results as the API hotels, so your customer sees one clean list and does not know which rooms come from where. You can usually choose whether your own contracts are shown first when the city and the dates match.
The honest limit is that for those hotels you are the supplier. Nothing checks live availability for you, so if you forget to close out dates or update your allocation, you can take a booking the property cannot honour. The system will issue the voucher and record the booking in your reports, but it is you who sends the reservation to the hotel and chases the confirmation, and the same is true for amendments and cancellations.
If this is something you want to do, tell us which properties you have contracts for and start by loading just one, then run a test search for its city to see how it sits next to the API results before you add the rest.