I have a similar question about a home number that I want to port to Skype. Surprised that no one from Skype has responded to your question.
Can I use my home or another existing number as my Online Number?
Unfortunately not; when you purchase an Online Number, you will need to choose one from a pre-selected range provided with us. However, you can choose your favourite number from that range. You can search for your preferred number using the Enter your favourite combination search box, or by clicking on Show ten new suggestions to scroll through the available options.
I recall reading a while back that porting an existing number to Skype was available or was going to be available. If a Skype representative is reading this, please clarify question.