You shouldn't be installing from disk if you've purchased DLC - Steam isn't set up to do partial installs or deactivate DLC that you have purchased. What you should do (to conserve bandwidth and time) is once you have the game installed (fresh install, presumably) with the HRTP, use the Steam option to "Backup Game Files." This will create an archive that installs the game and HRTP from a local archive and the only thing that will be downloaded is any updates that are newer than when the archive was created.
I suppose if you need to achieve a fresh install, you could install from the disk, then copy the esp and esm files for the HRTP to the data folder before authenticating the game and checking for updates. If Steam starts the download of the files anyways, you can abort the download, copy the files over to the data directory, then verify the integrity of the game cache under the "local files" tab of the Steam game settings. The HRTP files should pass the verification and won't need to be downloaded.