It's possible, but we don't make any guarantees if we don't have control over the DNS settings. Note that we can add DNS entries for you, or even give you access to the custom DNS control panel so that you can add your own records as needed, so you might think about transferring it over to us.
We have been meaning to add a feature so that you can be emailed if your DNS information changes, but we haven't added this yet; if you do decide to continue hosting your DNS elsewhere, you'd be well advised to setup some sort of script to automatically check and see if anything has changed on a daily basis. IP addresses don't tend to change often with our system, but (again), we don't make any promises that they won't.
As long as your entries are pointing to the correct shared IP, your site should work. It is better, though, to add a unique IP address if you're using 3rd party DNS; there's again no guarantee that the IP won't change, but it's a lot less likely that it will.