I don't understand what you are trying to do here.
You do not need to import any computer objects from AD to TMG. Actually I advice against it. The purpose of computer objects, computer sets etc in TMG is to use them as building blocks for rules. I don't think you need to create rule for all 600 objects?
A computer object in TMG is merely a reference to an IP-address which you name something that makes it easier for you to keep track of it. This can be the same as the computer name but it does not have to. Furthermore, I assume that your clients get IP addresses
assigned by DHCP, there's no function in TMG that updates a computer objects IP address should it change (which it will...).