Javascript - how to add items to a SELECT tag in Netscape

I know this question is a little offtopic but, please...
My problem is:
I have two SELECT lists in my page, I want to select an item from the first list and then push a button to add it in the second list.
I can do that easily in IE but i don't seem to manage to do it in Netscape.

Any idea how can I do it or where I can find a good online help for Netscape's Javascript implementation?

Thanks