I've read in different places that you can mix skills. For example the Nightblade class with it's three sub-classes, Assassination, Shadow, Siphoning. If I pick Assassination with it's skill line, but want to add a skill from the Shadow skill line, how is this possible ?

You put a point into the first skill of te Shadow line. Then put it on your hotbar and you'll get Shadow skill xp.

Those are not subclasses, those are 3 skill lines of the same class. A NB has access to all three.

• If you are a Nightblade, you have access to all three Nightblade skill lines (Assassination, Shadow, Siphoning). You can put skill points into one, two or all three of those lines as you please.
• You will not be able to pick up any of the other 9 skill lines for the three remaining classes (Sorc, Dragonknight, Templar)

good to know, thanks for the info