Depending on how you are evaluating your expressions - and you don't say so we have to guess - you will either need to rework your parser or actually implement your own. If you are relying on effectively compiling and running expressions in C# to solve this, then you are on a loser from teh start - you can't modify the compiler.
You could however take something like this:
A Tiny Expression Evaluator[
^] and tweak it to allow your (rather non-standard) input format.
Or just tell your users to add an operator...