Yes, but by building HS2-only stations separated from the existing infrastructure, you're missing an opportunity to integrate the systems and to address deficiencies in the current infrastructure. Of course this would be a more expensive approach than an HS2-only station, which is one reason why I feel in many cases there is merit in treating the building of new lines and the provision or upgrading of stations as separate projects.
The two things in my opinion go hand in hand - new lines and increase in capacity on current lines with interchange between the two - Cost will be a big factor in the last part. One of the dangers that I see is people are too focused on current journeys and not the potential to open up new journeys and new combinations of journeys. It may need a change of line but if its frequent, reliable and timed properly then its not a problem.