Theoretically, yes. However you'll notice that some turbos come with an internal wastegate, and some require an external one; I would argue it's a bit of a waste to install an external wastegate on a turbo that has an internal one already; you're going to need to keep it shut if you want to use the external one.
I don't know where the best place to install one is, but obviously it needs to be before the turbo somewhere; and I'd guess that a point where the manifold branches meet, which is probably right infront of the turbo. On a manifold designed for it, there may even be a dedicated runner.
I'm sure someone with more technical knowledge will correct me if I've got this wrong. - The advantages are that there is no limit on the size of an external wastegate, so you can have one that's bigger than the internal version. That also means you can run a turbo with a smaller A/R on the turbine (because you don't need to make the space for the wastegate). They also give you more precise boost control, which is often useful in high boost / high power applications.