Basically, yes, you CAN throw in an out of key chord. It can add spice, surprise - or a whole host of other interesting effects if chosen carefully.
Most music styles frequently use 'out of key' chords. Very simple songs such as nursery rhymes rarely use anything out of key - Folk and C&W do sometimes, but not so often. Jazz does it all the time. I don't know any jazz song that stays in key - It wouldn't sound jazzy if it did. Even simple Blues uses out of key chords.
By the way, D7 is very much an IN KEY chord (of G major). What made you think it wasn't?
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