Can this be made with sourdough starter instead of yeast?

Yes, you can! I don't know what the hydration of your starter is, but it ultimately shouldn't matter with this recipe, which is extremely forgiving - you have a lot of latitude with the ratios here, given that you are not folding or shaping this dough, and it has a fairly high hydration to begin with. You literally pour the dough out of the bowl onto the sheet pan once the dough is risen! I've made this focaccia many times. (I always add at least a tablespoon of chopped fresh rosemary or 2 teaspoons of dried rosemary, crumbled, to the batter. I recommend that!)

I would simply add 100 grams of sourdough starter to the dough. Expect it to need a bit more time to rise, but again, don't worry about it too much.

Oh, one more thing. This focaccia makes excellent croutons! ;o)