Can I pierce my belly button after a tummy tuck?

I wouldn't recommend doing an invasive procedure to your tummy, as it is not yet healed enough.  If you get an infection, you could loos a lot of tissue, at least theoretically. 

I assume you have a hernia based on you Q.  This is odd, as it should be handled at time of tummy tuck.

I fully agree with Dr. Placik. The Blood supply to the skin around the belly button stalk is not normal at only 6 weeks after surgery and in many women the blood supply of the belly button column itself is not as yet normal. ANY injury to either area may result in healing difficulty and or infection which may worsen and result in loss of a patch of the skin around the belly button or the belly button itself.

Giving the area 6-8 months to heal and reform its blood supply increases the margin of safety for such procedures.

The same holds for umbilical hernias. The hernia should have been fixed before or during your Tummy Tuck surgery. At this point, unless it bothers you or becomes a surgical emergency, I would give the tummy tuck time to heal before going in again.