Can an American expatriate contibute to a Roth 401k offered by his American company?

I work for an American company in Korea and my company is now offering a Roth 401k. Since I don't pay taxes on the $83,500 with the foreign tax exclusion when I file my taxes, if I contribute the Roth 401k what will the issues be if any?

I am sure your company has someone in the HR department who can answer this question in detail but likely you can contribute. If you are actually an expatriot the fact that you live in Korea is irrelevant.

For tax purposes an "expat" is someone who has given up their US citizenship and doesn't pay taxes.

You are merely a US citizen living and working abroad. However, if you exclude all of your income from taxes using form 2555, you do not have any earned income. No earned income, no Roth.