If I'm remembering correctly, Simply Lemon is just lemon juice, water, and sugar.
SD Slim has a fantastic recipe for a Lemon Lime Hefeweizen (more of a shandy) that adds a jug of simply lime. I've made it before using that method with good results, so I don't see why you couldn't.
Just be careful when you add it. Some people reported a TON of krausen with Slim's recipe, but that could be more from the yeast than the Simply Lime. For that reason he recommends to add it after high krausen. YMMV.