I think you need to be more specific. Is this what you want:

on your old site you have a page called:
mysite.com/about

and you want this to be mysite.com/about

You can do this by just creating that same page and then edit and go into properties then click the tab page paths and locations. You can then name the url of this page.

Keep in mind you must have pretty urls enabled to do this (in sitewide settings in the dashboard)

Is this what you want?

No you cannot. You are going to have to create pages in C5 and paste in your content from the old site that way. This is true for every CMS out there, because they store content in a database and not in separate pages like your existing static site does.

There is a free addon in the marketplace that lets you create many pages at once:
http://www.concrete5.org/marketplace/addons/add-multiple-pages/...
...this should speed up the process (but you'll still need to paste in the old content after adding the pages).