You can try to dread just a few locks if you want, but dreads tend to grab any other loose hair that's around it and dread it too. If you're quite conscientious about separating the regular hair from the locks when that happens, you could probably do it. Washing is not a problem, but conditioners are, because they untangle new dreads. See the website and above for washing instructions for dreads.
See the Shampoo and Dreadlock Care pages for washing instructions for dreads.