I want to say "Yesterday, because I was hungry, I ordered a pizza", and I want to report the event not in the historical present, but looking back on it.

There are two options:

wa'Hu' jIghungmo', pItSa' vIvun

wa'Hu' jIghungmo', pItSa' vIvunpu'

Since I'm looking back on the event, the obvious choice must be {wa'Hu' jIghungmo', pItSa' vIvunpu'}.

But isn't it strange saying "Yesterday, because I am hungry, I have ordered a pizza"?

Is using/combining aspect with no aspect something which we can do? Or is it that in occasions as the above, the only choice is to report the event in the historical present?

It's not strange at all. Yesterday, while a particular state was in effect, I performed a complete action.

Would you have a problem with wa'Hu' jIloStaHvIS, pItSa' vIvunpu'? I wouldn't. ghung and loStaH are equally imperfective.

It would be weird not to include the -pu' on vun, because wa'Hu' pItSa' vIvun implies that what you're describing is not a complete action. It asks you to mentally occupy a moment when the ordering happens but does not express that the ordering was a complete act. To do this in Klingon's equivalent of the historical present is fine, because to speak this way is to mentally occupy moment after moment as they are described (the way paq'batlh is written). To report the action after the fact this way would be weird.

(Technically speaking, Klingon does not have a historical present, because it doesn't have tense. I don't know what to call it when Klingon does this. Historical imperfective? Narrative mode? It's not really important; just recognize that it's not technically historical present.)

-- 
SuStel
http://trimboli.name