Using present participle and past tense of an acronym
Solution 1:
I'm pretty sure there aren't any rules concerning this, it's something that is fairly new to English and, as far as I am aware, it is colloquial. Regardless, I think that it is generally acceptable to put the acronym in upper case ("NAT") + the standard ending for the conjugation you want ("ing"), without accommodating for the more specific rules regarding the conjugation. In more formal documents, I generally manipulate the rest of the sentence to accommodate the use of the acronym without any modification (e.g. "I was NATing" becomes "I was using NAT").