This is a frequently asked question for customers, and the answer is "everything" but Zapier is limited to the integration built on it.
So maybe you want to add a note in Hubspot CRM, but in the current Hubspot CRM zapier integration there is no action to do that.
So the short answer would be, no I cannot add a note inside a deal in Hubspot CRM (this happens very frequently)
But, We are developers, and we can send information directly to an API (if it exists).
In this case, there is an API for Hubspot, and there is an endpoint named Engagement where I can create information on it.
So I can use webhooks or code to consume the API and achieve your desired result.
If there isn´t a zapier action/trigger or if there is no API endpoint, it won´t be possible to create your integration.