I forgot to make a screenshot, but when you buy Mission in an Outpost turn, you should be able to choose the order and you indeed can. I got offered two buttons to tell which turn should happen first. But both buttons had no text, so I had to guess. I was playing in English if that is somehow relevant. Let me know if you need more info.
I recreated the situation and made a screenshot.
The situation is however slightly different than what you described: You have to play Outpost and buy Mission on the same turn. That's what creates the 2 extra turns and you have to decide between them. Buying Mission on an Outpost turn is normally useless, because (without umentioned extra-turn effects) the Outpost turn follows one of your regular turns (the turn you played the Outpost), and hence the condition on Mission "If the previous turn wasn't yours" is not fullfilled.