Is it possible to write a proposal to get a junior professor position in Germany, when existing openings seem far from my research interests?
Stephan's answer is generally correct, but I get the feeling that this is not what the OP has in mind:


  Is it possible to write a proposal to ask for a professorship position?


I take this to mean whether it makes sense to essentially do a "blind application" for a professor position (i.e., send to a university that is not currently conducting a suitable search an application to the extend of "here are my credentials and a research proposal, please create me a position"). The answer to this question is a pretty resounding no. This will only make you seem out of touch with how academia works.