Just create a new module and make your new property in that module. When creating the property, be sure to specify that it's global in scope (i.e., not "private" or "protected"). It will then be accessible everywhere.
Check out the REALbasic Tutorial -- it covers all this in great depth.
Also, you could place your properties in the App class to make them global, but then you have to refer to them as something like App.MyPropertyName or some such.
And be sure to use globals as sparingly as possible.
HTH,
-- Steve