As you already noticed there is unfortunately no way to check if a variable is already defined.
You would have to write a program which saves the worksheet and scans the xml-file in the saved archive (a mcdx-file is just a renamed zip archive) for the existence of a variable definition.
Also Valerys suggestion does not help as Prime would reject the call to the function if you provide an undefined variable as argument and the function so would be never called.
But if just checking if a (defined) variable has the "value" NaN is enough, your solution can be simplified a little bit by omitting the local variable y. Furthermore you could follow Freds advice to use the if-function instead of the programming if-statement. This has the benefit that the routine will also work in the limited Prime Express. The definition will be shorter that way, but also a little bit less clear.
