If you're importing a podcast from Fireside.fm, you'll want to contact their customer support to set up a feed redirect. You can contact Fireside support through their support portal:
https://fireside.zendesk.com/hc/en-us/requests/new
You'll want to have handy:
Your new Pinecast feed URL (you can find this on your podcast dashboard)
Your podcast name and existing RSS feed URL
The email address that you signed up for Fireside with