Will nanogen hair fibres or make up concealer work to cover fue scars 6 weeks after fue?

It is best to discuss the use of concealers with your doctor.  It probably would cover up the scars but your doctor (who knows you) can better guide you.

After one month most doctors would consider your transplant recovery finished. So there aren't really any normal activities/products that you shouldn't be using at 6 weeks- hair fibers and concealers are fine to use, unless your doctor has told you to wait longer than 6 weeks for them.

You can use any concealer once the crusts are off of your recipient area. Many people do this regularly if they used them before