Bitter orange peel, available at your brewstore if you have one though thats usually early in the process and not at bottling. But maybe you could very finely grate some fresh orange peel and mix it in with your priming sugar, you will just have increased sediment when you go to drink it. Not ideal but not the end of the world either.
Anyone think this guy could use some orange juice concentrate from the grocery store in place of priming sugar to impart orange-ness to it? I would let someone else weigh in on that, don't take it as advice but only an idea.
I just know I would avoid anything artificial because it can/would interfere with proper carbonation and possibly create off-flavors.
Brewing: Dixie Peach Wine, Trader Joe Blackberry Wine, Red Grape Pyment
Bottled: Simple Sweet Cider, 7 Berry Apple Cider, Sweet & Still Cider