Looking to extend L visa to a F visa in Shenzhen...can anybody help??

I am American and I currently have a China L visa that will expire in Sept. 2012. I have already extended this visa once at the PSB in Louhu, Shenzhen. They have informed me that it can't be extended anymore times. Is there a way to get a F visa for 1 year or 2 years? Are there some good reliable agencies in Shenzhen or Hk
that can provide this type of service? Any info will be greatly appreciated. Thank you.