It would be simple to have a small NX/Open program read the expression value and set the translucency of the object(s). The only problem is that you would then need some way to trigger the execution of the NX/Open code. That's one benefit of KF -- it's fairly easy/natural to get KF code to run whenever it needs to, without you having to tell it.
So, as always, it depends what you're trying to do. If users are supposed to change some expression value to modify translucency, then it would be easier to have them interact with some little NX/Open program, instead. But if the translucency changes are all supposed to be automated/hidden/magic, then expressions and KF might be better.