Can I add instant yeast to sourdough dough that is not very active?
Yes, you can add instant yeast to a sourdough.  However, the fact that you used AP vs bread flour should not have much to do with the fermentation activity.  The different flours have different protein contents, which impact gluten development. Certainly, allowing it to ferment overnight is an option, but if you are short on time you can use an instant yeast to speed things up.