If i remember .NET Reflector correctly you should right click the class from the class navigation panel (the left one) and select the Reflexil -> Function
As far as I know there isn't an easy way to add the parameters.
You have to add those one by one from the reflexil window.
To change a function's code, open the reflexil panel and select the Instuctions tab.
Right click any of the instructions and press "Replace all with code"
Copy your code inside the function block of the newly opened window.
Press the compile button and then OK if everything compiled fine.
EDIT: I forgot to say you need to save the changes!
Right click the dll from the class navigation panel -> Reflexil -> Save As
The "Replace all with code" is still under development so you will (like me) experience a lot of compile errors.
Things I've come across are problems with inheritance, where you will have to change the class and function definitions in the "Replace all with code window" and enums are skipped for some reason in that same window.
You can just add the enum to make things work.
I'm probably missing something, but I imagine it requires a lot of work to re-add your methods on every update
Yes that is why I try to put as much code in a single function with a parameter linked to a switch statement.