Xml file produced from the entire web site or blog posts

Hi

as you know in the feedburner you can get a xml file from your site rss. but it depends on number of your site entries shown in rss feed that in some blog services you can set that( number of entries in rss)
now i need an online service or a solution that i can get a xml file from all entries of site not only from last number of site entries...like wordpress or other weblog services like mihanblog.com and etc.
in brief i need a solution to get a xml file from all posts of a non-wordpress blog or similar that has a rss feed i cant set its number.
i hope i could explain how i need. because my english ability is not very strong.
Thanks a lot.