Is it possible to make these substituting instant yeast?

To convert this recipe for hot cross buns to use yeast instead of sourdough starter, add 13 grams each of water and flour to the levain, to account for the started not used. Stir in 3 teaspoons / 9.3 grams of yeast. I'd use cold water in the levain, as the yeast will respond fairly quickly - no point in hurrying it along, as you will get a nicer flavor if you let the levain rise overnight. Then proceed with the recipe as drafted.
There is no hard and fast rule for how much yeast to use - a general rule of thumb is 2 1/4 teaspoon / 7 grams for every 3 - 4 cups / 260 - 480 grams of flour at sea level. If you use a little more, the dough will rise a little faster; use a little less, it will take a bit more time. ;o)

So glad to hear they turned out great! Yes, definitely possible with IY, I just don't know if there's a simple conversion to get you there (I never work with IY so it's hard for me to say)!