This isn't really a DNN specific question, but it is something that DNN folks face.  In general, one of the biggest hurdles for designers is the idea of following ( what may appear to them as ) a rigid formatting strucutre... by which I mean "panes".  The whole notion of having particular "zones" of content requires a little learning curve.

As for transferring an existing site... it depends entirely upon what that site does and how closely the behavior must be duplicated.  In most cases, it is possible to achieve near 100% visual duplication, although realistically... you'll want to change a few small things... like adding Site Search if you did not have it before.  Menuing systems can be a little harder to duplicate, I just finished a conversion for a friend of a friend ( I don't make a habit of this ) which required the use of a Panelbar menu which required a custom wrapper to fully duplicate their exisiting functionality.  In that particular case, it had to look the same to preserve investment in existing marketing literature.

Start by converting a skin... and itemizing the "functions" that the website needs to support.  Does it have forms?  Media?  Other interactive components?  Make sure you can duplicate what you need to in an acceptable manner, before you start.

Scott Willhite, Co-Founder DNN