Can I write on iPad using touch typing technique?
The onscreen iPad keyboard offers no way to (re)orient your fingers to the home keys, which makes touch typing nearly impossible in my experience. However, with practice many iPad owners become skilled at using the onscreen keyboard well. 

If you absolutely need to touch type longer documents, you can use a bluetooth keyboard with the iPad.

I rarely carry the external keyboard when I'm out and about, but if I plan to settle in to write I toss it in my bag and when traveling I keep one in my hotel room for evening writing marathons.