I am using a tumbler site (http://howmuchcanyouchugfoo.tumblr.com/tagged/podcast/rss) to populate a podcast on itunes, by way of feedburner. Despite enabling infinite scroll, Tumblr seems to only make the last 15 episodes available. I have tried adding ?num=40 to the url, but no luck.
The solution is to use either Feedcombine, Chimp Feedr, or Yahoo Pipes to merge the feeds into one single xml link as I learned here:
http://www.creativeramblings.com/3-tools-to-combine-multiple-feeds-into-one/
I tried all three sites and settled on Feedcombine as Chimp Feedr didn't seem to work on Feedburner, and Yahoo Pipes was displaying the posts in the wrong order.