It's a bit strange, usually we try not to conjugate foreign names in such cases. There are several ways to solve this. Example: You're meeting someone new and asking if it's him. Simple "Sam?" , "Tom?" , "Alice?", will suffice. Or "Czy to ty Sam?", literally " is it you Sam?. Most natural in my opinion.
Well, a sentence like "Jesteś Samem?" makes rather little sense because it's like "Are you a Sam?" instead of "Are you Sam?". And the fact that Sam is a foreign name is completely irrelevant, I also won't say "Jestem Markiem".
But then of course "Sam" undergoes declension like every other noun in other contexts, for example "Rozmawiam z Samem" = "I am talking with Sam".
As far as I remember, the male names that don't undergo declension are those that end with -u (in prononciation, so also "Hugh") and short names ending with -o, like Leo. Those ending with -e or -o are problematic because their declension often sounds weird to Polish people. Female names undergo declension only if they end with -a.
Without any context first thing that comes to mind is "alone here". With some context it could be either. Usually it would be with some indication of time eg. : So now your'e alone? "teraz jesteś sam". It would be little unnatural still, as we actually use "single" e.g. "Jeteś singlem?" or "Jesteś wolny/a" [Are you free?]
It is rather wrong, unless there's a situation in which "Are you a Mr. Sam?" makes sense in English. If a noun phrase doesn't make sense with an indefinite article in English, it wouldn't make sense in Instrumental in Polish.
As for how to say it... I don't think it's a common thing to say, I don't know how to put it in words. Maybe "Czy nazywa się pan Sam?" or just "Pan Sam?" on its own? Or maybe it's better to say "Szukam pana Sama" (I am looking for Mr. Sam) and let him let you know it's him.