Yes you can recruit them!
First you need kill Darkrai at Dark Crater. Then you can go back to their dungeons and fight to recruit them.
Some legendaries don't have their own dungeon, so you need a Challenge from them as you can see here is a list where you can find all legendaries and how recruit them!
Another thing you need to know are the Recruitment-affecting items.
Friend Bow increases chances by 10%.
Amber Tear increases chances by 24%.
Golden Mask increases chances by 24%.
The IQ Skill "Fast Friend" raises the recruitment rate by 1%. It's important to notice this 1% is crucial for recruiting Kecleon, which has a recruitment rate of -49.5% (-33.9% in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team), making it impossible to be recruited with only a high level Pokémon and a Golden Mask/Amber Tear.
Fast Friend is learned only by Pokémon in IQ Groups B and H.
Source 1
I'm also playing PMD Explorers of the Sky right now, started 2 days ago!
Hope this helps!
