You don't have space suits by default, you have to build them or find them on missions. If you are asking how to equip different armor, the armor menu in the craft screen will let you equip new armor.
Aye, I have space suits researched, I have them made, I HAVE THEM EQUIPPED EVEN! ...but, the mission doesn't know what kind of space suit armor is used in space missions, I'm guessing this is a bug in the rulesets? Same for undersea and the chiller.. it's funny getting into a diving suit to get naked in the water, again. >_> lol
While all that's said. Are underground missions, and base attacks supposed to be deckhand/runt only? Cause if I don't use either I go in naked.