There is feature request logged for this issue, so at some moment it will be implemented http://mantis.linuxmce.org/view.php?id=4013
Like a temporary solution, you consider installing one of the podcast fetching tools, like
podget - Podcast aggregrator/downloader optimized for cron
podracer - podcast aggregator/downloader
hpodder - Tool to scan and download podcasts (podcatcher)
and setup it download your *-casts to the place where LinuxMCE can find them and add the invocation of the tool to the cron for regular execution.