From B2evolution

Since 2.4.0-rc2, b2evolution has support for podcast. Just upload the mp3 file to your server, enter the URL of an mp3 file into 'Link to url:' and select 'Type: Podcast'. It will use a flash player on the page, and will add <enclosure> tags in your rss2 feed.

For versions lesser than 2.4.0-rc2, there is a hack which may accomplish this. You can read the details and developments in the following forum thread.

Another option is to just put a regular link to your mp3 file in a post and use a service like Feedburner, which will scan your normal feed and republish it, converting any links to mp3 files into RSS2 enclosures for a podcast.