Hi,
We have purchased a bundle of products, iClone 7, Character Creator 3, real skin, hair pack, makeup etc.
We are looking for the best product to edit/alter existing clothes (e.g. trouser length longer or shorter), and hopefully create clothes, either from scratch or use others and modify them.
What are the best products for thislength please?
Thanks
S 

I use opacity maps to alter the length of clothes. Here are some photos.
The avatar wearing short skirt and white top, opacity map did not work on the white top. 
I had to use CC3 edit mesh/face and soft selection. I highlighted the parts I did
not want to show and clicked on the hide button.
Opacity is just using  white on what you want to show and black on parts you
want to hide. Opacity map for tunic and rocker jeans are second and third pics. 
You can make opacity maps most of the time using the UV maps of the clothes.
The UV map is highlighted in the first pic. You click the little square and use a photo editor to open it.
The UV map of the rocker jeans is on top of my post.

Opacity maps can certainly make clothing shorter, apply holes or make lace out of existing clothing. MB is absolutely correct. There is a world of things one can do with modifying existing UV maps in a program like GIMP.

To make clothing longer however, this approach simply will not work and the CC native mesh editor is very limited in its functionality.  I believe that CC4 will begin to rectify this, but certainly CC3 is not particularly viable unless your mesh is very simple and you are very patient.

A neat way to do this is to put stuff into Blender where you can stretch and or add polys to your heart's content and even make new UV maps and then reimport into CC as an accessory that you then convert to cloth.  This depends very much on where you are getting the clothing models from. Realusion products can be very difficult to get into Blender unless you pay for export.  Any existing OBJ or FBX file from somewhere else is just a matter of clicking "import."

IMHO, Marvelous Designer is the best program for this though.  You can import your avatar and their ourfit directly into MD and have at altering the bits to your heart's content.