very easy… first import your MP3’s into your local song library within iTunes.
Select all the MP3’s you want to reclassify as Podcasts (and use the Podcast app).
Right click and “Get Info”
Go to the OPTIONS tab
select the “Media Kind” drop down and choose “Podcast”
all the selected MP3’s will now be moved over to Podcasts in your library.
You can now choose to sync them to your Apple device and use the Podcast app to listen.