Quote - i just couldn't find the extra bone in bertha bones setup... is there one in it ?

No, there isn't. Try bending Bertha's shins and you'll see the limitations of her rig - her feet will go through her skirt. The shins have no influence on the skirt at all, and it only works because it's so big and doesn't get in the way of many poses. Her rig is quite a good compromise, but she still can't sit down.

Quote - i do not want to create the skirt as a seperate object and import as 'prop'.
so, in this case, what'd be the best method to clothify it ?

maybe simply say.... how can i setup the collision detection for the skirt (rename as hip)  which is part of the character bodyparts ?

I've never seen this done before, but I don't know of any reason why a clothified part of a figure shouldn't collide against another non-clothified part of the same figure. The cloth room certainly allows you to set up a simulation that way, but I don't have a figure to try it with.

I'd do it this way, I think:

Build your figure as usual, and group the body parts; group the whole of the skirt to its own group, but don't add any bones to it.

Once you have the rigging for the figure set up, you can go into the cloth room. Pick the skirt as the thing to be clothified, and the legs (and probably the hip) as the things to be collided against. Then run the simulation as usual. If you have no experience of the cloth room whatsoever, it might be best to practice on some pre-existing clothing beforehand to get an idea of what's what.

Quote - any advise on how you do the morphing approach ?
i check the cloth room and couldn't find any morph tools .
and how do you link the morphs to the leg movement ?
thank you !

Once you've run a simulation on a particular leg position (sitting, walking, etc.) you can export the skirt in that position and then apply that as a morph target. Theoretically you should be able to spawn a morph in Poser, but there's some problem with that which I can't remember right now.

Just to re-state what I said earlier - you can't link the morphs to the leg movement. Well, I suppose you could, but it would be fantastically difficult. You need to control one thing (the skirt morph) from a series of joint parameters, and I have no idea how you might detect that the figure is sitting down and apply the "sit" morph, for example.

Really, for animation purposes, I suggest you forget the morph approach. If you really must have a long skirt, then the only viable approach is dynamic cloth, with all the computing time that that implies. Poser's rigging just isn't up to the job of doing it any other way.

Having said that, if anyone wants to prove me wrong, I'd be very happy to see what you did. :)