I've never heard of it at my agency so it probably varies on agency. Ask around

I am not sure how I would have negotiated before hire, but once I was on board, someone clued me in to a form I could submit to HR to document relevent previous experience. The information on this form changed the date that they used to determine leave accrual. So not only did I start at 6 hours leave accrual, I was bumped to 8 hours in less than a year based on that date.

Can be done, but don't assume you will get it. Your manager needs to go to bat for you, and some agencies are more stingy than others.

The basis is pretty much that if you are considered a significant enough hire, they will credit you years of prior experience in non-federal jobs towards your federal years of experience. So you'll need at least three years of non-federal work experience they can count for you to get you to the 6 hour threshold on day one.

http://www.opm.gov/policy-data-oversight/pay-leave/leave-administration/fact-sheets/creditable-service-for-annual-leave-accrual-for-non-federal-work-experience-and-experience-in-the-uniformed-service/

Yes I know someone who negotiated for 6 hours - mid-level position

Yes. If you are highly desired, they can give you 6 hours a pay period vs 4 hours a pay period. It's hard to get though. At my agency we need approval from senior management to do it. Normally it's done because the person has a much higher salary in the private sector and their talents are in demand.

Is the rate of leave accrual negotiable based on prior private sector experience upon entering the federal government? Or does everyone have to start at the lowest accrual rate? I undertstand that the steps within a GS level position are negotiable based on prior private sector experience and compensation, but I'm not sure about leave accrual.
If leave accrual rate is negotiable, like step, can you tell me on what basis I can request a higher accrual rate?