I've contemplated this exact situation.
This is what I plan for my team:
Mix C# with F#, this can be done by using C# for the majority of the code-base. Where heavy data processing is required, write the associated functions in F# and put it inside a dll, or reference it. Example hereExample here
Slowly re-factor your existing code-base in the above fashion.
Not all code has to be functional.
Get your team to learn the basics of Haskell, LISP over the weekends.
Get them to learn F#, by trying to solve Euler Project puzzles (that helped me a lot when I was learning F#). Again this should be something done say over the week end, or during work time if you want set a day aside for "training".