Is there an enhancing, slangish word to put after statements, like the Norwegian slang word "ass"?
When subtitling, this word is God's gift to you!
Here's your predicament as I see it: you are bound to render speech, presumably casual dialogue, in subtitles. A word that seems to be a cross between an epistemic intensifier and a filler keeps being repeated. It has no obvious counterpart in the target language, but a lot of good renderings exist. The main point appears to be to sound colloquial.
My advice: Do not try to translate the word, especially not by making a list of possible English renderings and alternating between them. Just translate the meaning. If the meaning and style (and not the original wording in Norwegian) call for "man" or "you see", etc., then by all means put it in (even if the source phrase does not contain "ass").
I'm sure you are already doing that, but "ass" keeps bothering you. It would be a serious problem in dubbing (as your audience would be lip-reading). With subtitles, you want to be minimalistic; you may need to put less than the original while being truthful to it. So personally I'd be very happy and try to put all the colloquial, buddy-buddy feel of the conversation into the wording of the phrase itself.
So
"Jeg vet ikke, ass."
No idea.
or (if you must)
Really, no idea.
In Canadian English we tack "eh?" at the end of sentences for a similar effect. It has (as you call it) a lazy or careless tone, and is similar to "don't you know?" in Irish English. For example, "It's windy today, eh?"
In Singapore English they use the word "lah" at the end of sentences for emphasis, but I think most people outside of Singapore and Malaysia will not understand what it means.
Does it have to be American slang? British street slang innit seems to fit well. It means "isn't that so?", or "right?" https://www.youtube.com/watch?v=a5MFpHz1Mj8 Other options, "yeah?" (in place of "right?") examples: "I've been talking about this for a while, yeah?" or "I've been talking about this for a while, innit?"