This is will work for Joomla 1.0.xx, as we knows, Joomla 1.0.xx component, module, or mambots has been unpublished in Joomla.org, so we will difficultly to find any component to showing all content items in Joomla 1.0.xx because by default it will only showing frontpage items on Joomla 1.0.xx RSS.

We need one hack on rss.php file in com_rss folder, so lets start browse folder :
/components/com_rss/
find and start open/edit file: rss.php
then find the following line (on line 234) :

. "\n INNER JOIN #__content_frontpage AS f ON f.content_id = a.id"

Commenting / disabled this line by adding two slash in front of that line, it will be like this after commenting:

//. "\n INNER JOIN #__content_frontpage AS f ON f.content_id = a.id"

save your change, and look at your RSS syndication now, it will showing all contents there.

If you confused about changing file source like above, you can download it here:
http://ninoholic.com/downloads
Just replace your old rss.php file with that, make sure you have backup first before doing that.