As far as i've seen, this isn't possible, at least as of 1308. I haven't really read through the 1311 documentation, though.
There are ways around it. You can use elements (regular or transient) and use these within the Business Object action.
Then within CustomBO.FooBar() (without parameters) you can use these elements.
Alternately, you can make a Reuse Library function, which DOES allow for parameters to be passed in - however, the Reuse Library functions don't let you raise messages as part of the function. I've used it in a few circumstances where i'd pass parameters in to a reuse function that would return a Code data type, and based on the code returned i'd take further action.
Generally i'd do this if i need to have validations in multiple locations within a SAP Business Object.
What does this requirement stem from? That'd help use recommend the best alternative.