Can I combine Exploding Kitten decks to allow for more players?
Absolutely. From the website, Exploding Kittens is a:

Party game for 2-5 players (up to 9 players with 2 decks)

I often combine the Original and NSFW versions, to play with more than 4 players. You just have to make sure that there is always exactly 1 Exploding Kitten card less than the number of players.