I run into problems similar to this when I port/compile opensource packages. Often it is enough to add a symbolic link from what is demanded to what is offered.
However, in the case of nim and ssl I fear they will be greater problems: you might be able to patch the server - patching the client is going to be more tedious.
My approach would be to get what I hope is a proper server setup, and then carefully make - and document - changes to a client so that I can create a script that can reconfigure clients to work with an openssh enabled nim server.
Have you considered using the new nimsh interface instead? Maybe that is sufficient for your needs!