Declaring a variable outside classDeclaration for use in form

I want to declare a variable that will be used on an existing AX form for AX 2012. I could put it in the classDeclaration of the form, but I would rather not modify the existing AX source code. Is there somewhere else where I could declare it? The variable will be accessed by control methods on the form. I tried looking into putting it in the datasource but this is not a class so it doesn't look possible.