Can I offer additional resources only after the main resource is added to cart?

Yes, you can do this. Open the dashboard and click on Resources and widgets. Create a resource group.

Add the additional resources you want to offer when booking the main resource to this group.

Open the widget and go to the Content panel. In the dropdown, select that the "Add more to order" button should open the group you created. Save your changes.

Add a Reaction to the reservation form for additional services. See how to do this here.

You can also change the Add more to order button to your own custom text. This helps customers understand what additional services they can reserve. See how to do this here.

See an example in action: