It is not currently possible to import notes into the feed using Hubspot native CSV import. However, you can do it using a third party solution - Import2 Wizard
The steps to import notes using Import2 Wizard are pretty easy:
Upload your CSV file, select “notes” as the type of data to import
Map the fields you want on the note itself (i.e. body/description of the note, timestamp) and the links to the records you want to associate the note to (i.e. contact, company, deal). You can use contact email to find if there is an existing contact in HubSpot to link it to (you don't have to use email, you can use any field on the contact, even custom ones). Here is documentation on how to do mappings in Hubspot.
I agree, this would be a great feature to be able to import notes on a contact or company directly into the notes on the timeline - without having to integrate anything OR create custom contact/company property...
I did not find a way to import notes into HubSpot CRM, but Import2 has a tool in Beta that they used on my data and it worked perfectly. 81,000 notes and there were about 3 skips, all of which were email addresses (the match field for upload) that had changed. You can't change the note title, but the body and dates came through seamlessly and there was no problem with API call limits, which has been a problem elsewhere. Definitely worth a go, unless HubSpot is going to change this glaring hole!