Asquared,
It is possible to "self study" for the ATP knowledge test (until August of this year) and if you are just taking the ATP checkride in a small airplane you can show up to an examiner and say, "Let's do the checkride." However, if you are getting an ATP type rating 61.157 (b) says:
(b) Aircraft type rating. Except as provided in paragraph (c) of this section,
a person who applies for an aircraft type rating to be added to an airline transport pilot certificate or applies for a type rating to be concurrently completed with an airline transport pilot certificate: