btchris,
yes thats basically what I want to do. I want to use only some of the addresses in the same wallet and export them to the webwallet but keep the rest of my btcs in that same wallet (spread out in different addresses).
I'm not aware of any way to do this without some risk. If your wallet file is ever compromised, even if it's password-protected by Armory, or even if it's a watching-only wallet, then whoever has both it and one of your private keys can gain access to all of them.
It may be less convenient to use new addresses with a web wallet, but it's the safer choice.
If you're 100% sure that nobody has or will gain access to your wallet, then it should be safe, but it's hard to be 100% sure...