VHDL "casting" a constant into a signal?
Just make the formal parameters constants (i.e. delete the word signal in the function declaration).

You're not driving them within the function anyway. (If you are, it isn't a function)

Then, when you call the function, the current value of each signal is passed as a constant to the function. 

This won't work if your function uses the signal's attributes (such as 'event or 'stable, but 'length etc which aren't specific to signals would be OK). If you DO need signal-specific attributes you'll need to add a signal anyway.