Can I add bonding to a composite veneer that was made in a lab?

If they were recently placed, maybe your dentist will redo them for you since you are not happy with the color.  He could bond composite to them, but composite is more prone to staining and weaker than a lab processed veneer.  To do this he would need to roughen up the lab composite veneer and then place bonding agent and then form and shape the composite.  A better alternative would be for your dentist to learn about EasySmile  Veneers(soon to be ProVeneers).  They are being introduced to dentists at the Greater New York Dental Convention that runs Nov.30 - Dec2.  I will be at the show I Sunday at the booth featuring ProVeneers to answer dentists questions.  With this system, your dentist can roughen the surface, shape  and create a very thin covering that is bonded to your existing composite veneer.  The new ProVeneer will be  whiter, stronger, and more stain resistant than if he were to add composite.   The ProVeneers also allow him to place a PreView Veneer so you can evaluate color and make sure you get the color you want before he bonds permanently to your tooth.  This is a short 1 visit procedure that can easily be learned by your dentist if he is already placing veneers.  

It is not a common procedure to bond composite to a veneer to change the color. Did you have an opportunity to look at the veneers before they were bonded? I would suggest returning to your dentist and discussing your concerns about the color. Normally it is necessary to remove the veneers and replace them for color changes.

direct bonding on veneers is not advisable. it will worsen the look and texture of veneer. so best option is re-veneering.