Sneak attack equipment upgrade

How does it work with PC's skill? With 7d6 sneak attack as a PC feat and +3d6 sneak attack as an item property, would I have 10d6?

Yes and no.

The bonus feat sneak attacks are of the rogue type of sneak attack, and do not stack with eachother.  Only the highest value is used.

Those bonus feats will only work for PCs without rogue levels, PCs with rogue levels that grant less sneak attack than the bonus feat itself, or PCs with the blackguard sneak attack, or the assassin's death attack.

Overall, the feats are powerful for non rogues, and pretty much useless for rogues.

I've found out that death attack from items stack with sneak attack well (but dc for paralyze is still counted as if you were asassin lvl1).

We don't have bonus death attack dice damage as an item property in Aenea though.