Is it possible to add elements to RSS Item?

Anything is possible - and in this case - as long as RSS supports what you want to do then you can do it, since ASP only processes a page and spits out HTML (or XML in this case)

I cant tell you how, as you havent given any info about what you are using, what you want to achieve etc.

A little more information about the RSS (is it custom or a pre-made application) and what you want to achieve will go along way.