Really, you can add them anytime.
Plants will actually use ammonia and nitrate as a source of nutrients. So, plants really cold help speed up your cycle.
You really don't need to add any fertilizer, unless you want to. Plants should be just fine and live well in a aquarium without the aid of ferts. After the plants are living well on their own, then you can go ahead and add ferts to help the thrive! But, you can add ferts from the beginning if you want, it won't hurt a thing.
So yeah, you can add plants right now if you want, the only thing is, plants need light to live. Since you are doing a fishless cycle, you are constantly adding ammonia to your tank. Well, ammonia + light = algae. Since you need you lights on for your plants, and you have a high level of ammonia in your tank, you might have some algae issues.
So, it is best not to have your lights on during a fishless cycle, I mean, you don't need them on anyways. The bacteria in your filter will grow the same if the light was on or off.
-FHM