Zurückkommen is a separable verb, so the particle goes to the last position. Ich komme zurück morgen sounds odd, but I can't state a rule for this by heart right now. There are some places where you can interpret the particle as an independent adverb, but it doesn't really work out. For an adverb (or an adverbial) you must be able to ask a proper question like Wie/Wohin/Warum komme ich morgen? When this doesn't work well, it's likely that it's not an adverb but a separated particle of a verb.