I am wondering, is it allowed for an 'item' tag in rss2 feed to have properties?

I need to add additional info to my articles in my RSS feed, like item's "weight" and "expiration", so I was thinking to adding these as properties of 'item', like this:
<item weight="2" expiration="10-11-2009">

all other usual tags here


Is this allowed or will it cause problems in RSS parsers? Is there a better way to do this?