I would say yes, i haven't done it yet but cant see why not, unless there are some sort of restrictions at your host.
if you mean the cpanel addon domain (that, as far as I can see, is just an additional site but all grouped under your main domain) yes, no problems I have used a lot of subdomains on "addon domains" you'll probably be able to access it even from subdomain.addondomain.maindomain.com
Thanks for your replies I've been putting off purchasing a particular domain because I would want to use subs with it, and I just didn't know if I could do that...I'm excited now to get my new domain and get started!! (I'm a newbie) We'll see how it goes...Oh, and the original domain name I purchased (the main one for the account, is just too general) I'm going to get a domain that is more specific, containing a keyword.. Thanks again.