You can't get it to actually count, because it's just a phrase comparing machine, but you can get it to say a phrase that has the numbers counted in it.
Simply say the list of numbers to it at various times. You can't do it twice in a row or it will bump out to the "are you repeating yourself" set of phrases.
Maybe say something like "You will count, one, two, three", then later, after you've done it several times, when you use a sentence with the word "count" in it, Hal will replace the word "you" with "I" and say, "I will count, one, two, three"