Podomatic does not have a user interface from which to set up a 301 redirect. Therefore, to migrate your feed, you must email Podomatic at email@example.com asking them to set up a 301 redirect from your Podomatic RSS feed to your RSS.com feed.
Here is an email template that you can use:
Hi Podomatic Support,
I'm moving my podcast to a new host provider and kindly request that you apply a redirect for the following show (Insert Your Podomatic Feed Here)
Can you please apply a 301 redirect to my new RSS.com feed at (insert your RSS feed here, for example, https://media.rss.com/myRSSfeed/feed.xml)
Please let me know if you need any more information
Thanks in advance.