I'm not too sure how it happens. However, my Dad knows someone who is going through training at the mo. The guy really impressed the PTI's during the training with his running. So they told him to focus on his upper body so that he could start working towards becoming a PTI.
I'm not sure though if you have to attain a certain rank to become a PTI. I know in the army you have too.
To the best of my knowledge, you have to have completed 2 years general duties after completing training, get a written report recommending (Grade 'B' RORRS Report) you as a PTI & have completed Junior Command Course, becomming eligible for promtion to Corporal, before applying for PTI's course.
Happy to stand corrected, if it's different, but being a qualified civilian PTI before you join will make little difference when you first join I'm afraid.