Let me try and answer some of your questions. First the panels you are looking at that are rated 100A simply means you can use them for any application up to 100A. You can for example add a 60A breaker to your existing panel and protect the new subpanel with a 100A rating. It is not so much what the panel is rated as what the protection is rated.
You can use other methods other than conduit but first you need to select what type of method you prefer to use and that must be permitted as outlined in Chapter 300 of the NEC "Wiring Methods". So you need to select a method then follow the installation directions as outlined in that particular selection. As an example, let's say you want to use type NM (romex) to connect the new sub panel. That would be Article 334 in the NEC. It will have uses permitted with exceptions and uses not permitted and give installation rules you must follow depending on where these panels are and the type of environment they are in.
I would like to suggest that it may be a better idea to replace and upgrade the existing panel. I believe this would be a better method and give you a better value added rather than several subfed panels.