I'm in China on a 3 month f visa, it will expire soon and I would like to get another one.

Eric, in my opinion, if the agency says you can only get 60-day L type from Hong Kong, then it must be true. At present, if you don't own residence status in Hong Kong, it will be hard to do the application there. Anyway, another option is trying to extend your current F type within mainland China. Perhaps this is the most desirable solution.