So the answer to the question you asked is, YES, there is a way to IMPORT your stuff from PinPics - but, as you discovered it only adds new stuff, it will not delete stuff you have in DPP.
What you are REALLY looking for (in computer geek terms) is a way to SYNCHRONIZE your DPP pins with your PP pins, so you only have to maintain your collection one place. I know there are pins in DPP that don't exist in PP, so there are some inherent issues with a synch function like that... Anything you add in DPP that doesn't exist in PP would be removed once you synch, any changes you make in DPP but don't make the corresponding changes in PP would be reset on a synch...
I can't really speak for the site's developer(s), but in my opinion, the "import" feature was supposed to allow you to easily transport your collection, the thought being you would do all your collection maintenance in DPP going forward, and would just leave PP behind.
Of course, that's not quite feasible yet, since DPP doesn't have the "trade assistant" functionality, and a few other cool features I am sure are coming soon (on the developer's list of to-do items).
Until then, dropping items from your trade list, or your collection, is fairly quick in DPP... You can search pins for a description (Say, "world of color") and then using that page that only has the pins you want, click on one, remove from my trades, click next pin (and this works GREAT) it takes you to the next pin from the search results, not the next pin in the database!! I have added and removed 15-20 pins at a time in a couple of minutes, this way.