There are a number of good drawing apps for the iPad, some of them, like Sketchbook, have PC versions that you can move the drawing back and forth between.
However, do not expect the kind of accuracy and responsiveness you get with your WACOM. The iPad's screen is capacitive.. This is a lot less accurate than your WACOM tablet and does not directly support pressure sensitive.
To get around this there are several companies that make active styluses for the iPad that are a bit more accurate than your finger, and add pressure sensitive. These usually pair with your iPad using Bluetooth and only work (with all features) with compatible apps. The Jot Adonit styluses are the ones I've read the most about, but there are others.
It's all compromises, but it is doable, and a lot of fantastic art has been produced on the iPad.
It occurs to me that what you might be asking for is a tablet peripheral to work with your iPad. As far as I know there is no such device; though there might be something in the
Hacking forum that would allow you to pair a drawing tablet with a
jailbroken iPad, I doubt you'll find any apps that support it properly. But I could be wrong.