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

Yes, you can do it. Create a resource group.
In this group, put the resources that you want to offer when booking the main resource.
Open the widget. In the For adding to an order use dropdown select 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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