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

Yes, you can do it. Open the dashboard and click Resources and widgets. Create a resource group.
In this group, put the additional resources that you want to offer when booking the main resource.
Open the widget and go to the Content panel. Select from the dropdown that "Add more to order" button should open the created group. Save.
Add a Reaction on 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 text. This way customers will understand what other additional services they can reserve. See how to do this here.
See the example in action:
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