Is it possible to have existing knowledge base articles pop-up before someone submits a ticket on that topic?

Once you have populated your knowledge base articles, you can setup your support portal to "auto-suggest" solutions that are in line with a customers' query(Ticket Subject) when they are submitting a ticket.

If you navigate to Admin > Channels > Portal --> Settings tab, you would see an option called "Auto-suggest solutions while creating a new ticket", available under "User Permissions for portal" --> Who can submit a new ticket on portal.

You could turn this option on and each time when the customer starts submitting a ticket, based on the text added to the Subject, corresponding Solution Articles would be auto-suggested.