Can I use body hair to cover the scar and fill up in between the fallen transplanted hair?

Many things are possible with hair transplant surgery but you need an examination to make sure it is right for you.  A doctor needs to see you and examine your scalp.  You need to give your doctor clear goals and understand the limitations of surgery.

Going for hair transplant in the scar and to fill in the failed hair transplant areas is an option which works well when done by an experienced surgeon. Another good option to hide the scar is scalp micro pigmentation.

There are few things to consider:If the FUT scar is wider than usual (more than 1-2mm in thickness) then it deserves a repair. If the procedure was not done by an expert (which is usually the case)  then doing a scar revision with tricophytic closure may improve the scar significantly. If the widened scar is thought to be due to unlucky tissue character rather than poor surgical technique, then transplanting the scar with scalp hair or body hair may be an option to camouflage the scar. If body hair is considered then beard hair is the most effective due to its high caliber and longer anagen phase in the hair cycle.Body hair may be used to improve hair coverage in some situations, but its effectivity should not be overemphasised. Body hair other than beard hair is more unpredictable when it comes to yield. They are fine and follicular units mostly consist of single hairs. Beard hair is more effective but the donor capacity is limited. A general rule is that transplanting body hair is not nearly as effective as scalp hair and if a result hasn't been achieved by using scalp hair, then body hair doesn't promise a miracle. 

There are many ways to treat an unsightly FUT scar.  Transplanting into the scar with scalp or body hair is certainly an option.  Another option is tattooing the area, or doing a combination of tattooing and transplant.  A second FUT can also be considered, but the use of A-cell and growth factors at the wound site would be recommended to decrease the risk of scarring again.  I'm a big proponent of growth factors, PRP (platelet rich plasma) and SVF (stromal vascular fraction - which has stem cells from your own fat tissue) to help grow hair and to help treat scaring.  Bottom line, is that you have a ton of options if you are willing to pursue them.  

It is a good idea to have a consultation with an experienced hair transplant doctor in person  to review your options.  There are many options available but it is best for you to be seen to customize the plan to your needs and concerns.

Your problem is what we call depleted donor area with scarring and this is best treated with Scalp Micropigmentation which works very well to make the donor area look decent and cover the scars. This same procedure also works very well to fill in the failed hair transplant areas and we have people coming from all over the world to see us for this treatment.  See web reference below