I know CD is not designed to do this but is there a way to use the "computername" function in an answer file to not only name the computer but add it to an AD group?
Using sysprep tags, yes.
You can use this to maybe get you in the right direction.
Yes, you should be able modify any existing sysprep entry. There are some limitations though, which I have addressed and will be available in the next version. Each situation varies. What is the name of the sysprep field? Is it one line? Does it need updated with a variable or static entry?