Does the HV10 have auto built in ND filter or does it just adjust shutter and aperture?

Canon calls it "ND gradation" feature. I believe it has 2 different ND filters in the optical stream. No direct user control over them however I think they can be indirectly controlled via the exposure adjustment feature in the program modes.

great camera, but it needs a polarizer or additional ND filter. skies blow out easily.

So it does have ND's built in, with no access to change?

Much info here:
http://www.dvinfo.net/conf/showthread.php?t=81516

Including a link to the original patent. The ND filter is the 'pie-chart'-looking piece of (semi-)transparent glass or plastic. Used ND factors are 0, 2 and 4.

I think that adding another ND filter will only cause the camera to 'open up' by either decreasing its own ND, increasing iris opening or increasing shutter time.

So unless the cam is at max ND, max F and min shutter, the extra ND will do nothing........I would say.

regs
Pieter