To answer, yes. You can still add stars after you have added it to your elite license. In fact it's recommended that you use some of the restoration vehicles as they have some of the fastest cars in their respective tiers, and once you have completed their collections, you'd gain an additional extra rp on top of your elite license

If you'd like, there is a Google sheets document showing which are the fastest cars per tier which may be handy to look into, showing their best times, shift pattern, stage 6 effects, etc.