Short answer
HubSpot's KB: import from Freshdesk, Help Scout, Zendesk or Intercom, or a CSV. Choosing Import all content as Draft keeps articles hidden until published. Professional allows one knowledge base with up to 2,000 articles.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Check article counts against your tier's limit before importing.
3. Related
Multiple knowledge bases and styling.