I am negotiating an offer at a company that has a defined benefit retirement plan. I have 6 years of experience in my field. Is it reasonable to ask that I be placed in the plan as if I had 6 years of service at the company?

Depends on what the plan offers.
To push for inclusion at that level is rather presumptuous.

To be honest, a mere 6 years experience in a field still has a lot to learn.
I've been in my current professional field for the last 15 years and previous to that for 10 years, yet I would not consider a jump in benefits as I am still the "New Boy" in that company.

Your experience got you the job, not the benefits as they are earnt.

I was not implying that my six years of experience means I have nothing to learn, you were assuming a lot with that answer which did not actually address my question. I asked if years of service in a defined benefit plan was something that could be negotiated.

You can of course always ask, I doubt they will offer it, but all they can say is no if you ask.

Generally its not. Defined Benefit plans are governed by a plan document. And companies do not have flexibility about eligibility.