No there is not.
Is there a default value that can reasonably be applied for z (perhaps calculated by x and y) ? If there is, then you can code
function f(x,y,z)
if ~exists('z','var')
z = ... %the default value
end
If this makes sense then you can make the changeover gradually, first doing the places were a different z is important, and then for consistency going back and filling in the ones were it is less important.
In the case where you know the name the variable would have in the calling code, you could use something like
function f(x,y,z)
if ~exists('z','var')
try
z = evalin('caller','z');
catch
z = ... %some default
end
end
I do not really comment this, but if it gets you through a crunch and you are sure you will be able to clean up the code afterwards... Note that if you just "intend" to clean up the code afterwards that chances are the code will stay warped, as there are always more things to do...