Is it possible to create a NFA that accepts only n*"a" or n*"b" inputs?
Yes it is.
Considering regular expression and NFA have the same power (source here), your question amounts to asking if there is such a regular expression.
So the regular expression would be /a*|b*/.
As for a NFA doing such a thing, here is an article describing how to go from regex to NFA.
In the article about regex to NFA, there is a section at the end explaining about metacharacter \$ for end of string. An actual regex for your problem would then be /^(a*|b*)$/ .