I don't think it's a simple upgrade... if your car has the 6 speaker set up - it's not worth trying to mess with the DSP Carver amp - you'd be far better off using an aftermarket amp to power any additional speakers.
In my opinion however, it IS still worth buying the Sub speakers and mounts/frame - because you could use said aftermarket amp to drive them - which is exactly what I did 8)
I have however already switched my head unit, which provides RCA outputs, so I simply ran the RCAs to a little Pioneer amp I put in the boot, then from Amp to speakers mounted behind seats where the cubby holes are. The cubby hole just pull out - use a trim tool to lever them and they'll unclip in a few places and slide out.
The mounting holes are there already for the subs in their frames - then clip the new grills (as seen in OPs pic) over the top.
With all the aftermarket options, it's far better to have the 6 speaker set up and add these subs, but use your own amp etc than it is to have the DSP system and change things. The DSP (10 speaker setup) needs complete rewiring of the Audio system should you want a different head unit etc - so yeah, personally, I wouldn't bother with Carver amp - there's no easy upgrade, and it's simply just not worth it. - But again, the Subs/Speakers that mount behind the seats are defo worth grabbing. From memory I paid £100 for mine from a fellow forum member some time ago - that included the grills, speakers, foam & the mounting panels. The speakers were NOT the proper subs though from memory (and I knew that when buying) so £150 is a fair price IMO if that's the Subs, mounting panel, grills and foam