I am a hobby historian who sometimes publishes historical peer reviewed papers on the side. I was wondering if it is possible to bundle these together at some point into a phd thesis? Do the papers needs to be connected to the university or can you just appreach a prof and write a summary of he is willing to 'sponsor' you?
Individual schools will have their own policies, with a strong influence by country of location. In the United States, for example, PhD programs involve years of coursework and most likely comprehensive exams before you even propose your dissertation topic. Some European countries (the UK in particular) have thesis-only PhD programs, with no coursework.
The thing you need to understand is a PhD is not a vanity degree. A PhD by publication-after-the-fact is the equivalent of an MFA for the Nobel Prize in Lit winner. The point is to provide a working, successful, academically-respected academic with the credential for the job they're already doing.
You apply to the school, apply to work with a professor, and IF you get in, you pay the school a truckload of cash.
Pretty much what others have said, but in the US, there are some universities that may have very small doctoral programs in history where the faculty would be willing to work with an already published amateur historian to help them reshape their existing work into a doctoral dissertation in a way that would leave intact a significant amount of what the candidate had already written. But there's no way to get around the fact that all doctoral programs involve training, coursework and a mandatory amount of time investment. No one but a diploma mill is going to simply accept a manuscript and a pile of cash and hand over a degree.