Last time I checked. The only receiver that DN would allow you to add to an account that has a Hopper is the 211/411 series receiver if you wanted to only pay the extra $7/month additional receiver fee. While the DVR should work with the 3rd output from the antenna, you may or may not be able to add it to the same account.
You certainly can do 2 Hoppers since a standard household installation requiring separate programming for more than 3 TV's would require a second Hopper (or now a Hopper and a super Joey but that's not a stand-alone receiver.