If you have a schedule C prepared elsewhere, there is no way to attach it to a tax return in turobtax. Turbotax is designed to have you enter all your business info and build your return from the ground up.
If you bought Turbotax Deluxe version on a CD or download to install on your own computer, you could manually create a schedule C by copying the numbers off the one you have (maybe prepared by an accountant) using direct number entry in Forms mode. However, using Forms mode voids the accuracy guarantee, and Forms mode is not available in any version of Turbotax Online. (Also, it will be a bit tricky since most number on Schedule C will flow there from another worksheet and you will have to find the worksheets to enter the numbers to build your Schedule C by direct manual entry.