No feeds are not searchable using the site search and I have no immediate plans to try and make it searchable. To me feeds are transient data and it does not make a lot of sense to index it into the site search index. That will make the index files grow large with no longer active data.
Since we cache the feed antries in the db it may be possible to do database based search but implementing that in all the db platforms we support would be challenging if even possible in all of them. A lot of work for little bang doe snot interest me right now.
If at some point I were to try and implement a search feature in the feed manager I would probably make it separate from the site search and make it use google or bing to search the sites whose feeds are listed in the feed manager.