Firstly AFAIK you can do some stuff through the AWS console, but I'm not sure what the limitations are. Personally I have never used it (I just use the Hub for server management) so I can't really give you a lot of info.

Secondly, I have no idea how you would be able to add an existing instance to the Hub. Probably the simplest, easiest way to do it, is use TKLBAM to migrate to a fresh instance. (make sure everything works 100% before you turn off your old one).

You can "watch" your instances in the AWS console, just be sure that you are looking at the correct region as the AWS console does not consolidate instances from all regions as the Hub does. You can change regions using the dropdown box located top-left IIRC.

Regarding your second question, there currently is no way to import instances into the Hub.