I know that there was an event in 2009 that allowed people to become zombies and i also believe there are some zombie events every year, but do those events just give you a zombie skin or does it allow you to shop in the zombie den as well

You will only have access to the zombie den if you're a zombie

okay. once you are a zombie, can you access all the zombie den items from the previous years

unfortunately no, you can only have access to the items from that year and forward. EX: a 2012 zombie can only have access to 2012 and 2013 items.

okay thanks a buncvh!

If you are looking to get those older zombie items there are many zombies who offer to buy them for users.