This is really a comment to the above answer, but is too long for stackexchange.
I got @nthonygreen's answer to work, but it was surprisingly not-obvious how to do it.
The first mistake I made was: I had my iPhone connected to my computer, and iTunes was displaying the phone's stuff in the left panel; the FIle > Library > Export menu was greyed out. It took me a long time to realize that I needed to click an icon in the upper-left that showed me podcasts in general, as opposed to podcasts on the phone; then I could export.
Then importing was tricky! I wound up emailing the opml file to myself, and clicking the attachment in the mail app ... this displayed the raw XML full-screen, with no obvious way to do anything with it (like get Overcast to import it, which is what I ultimately wanted). Eventually I noticed that if I tapped on the screen, a menu bar appeared at the top of the screen, and it had an "export" icon (a square with an arrow pointing up from its center); tapping that showed me a bunch of icons, and I was able to scroll them to reveal Overcast ... which happily imported it. Phew!