Is there a procedure to repair a softball sized muscular deformity in my left buttock, as the result of blunt trauma? (Photo)

You can use dermal filler or fat grafting to fill in the defect in buttock region.  If you are looking for a non-surgical option, I would consider Sculptra. This would be the ideal product for someone who does not have
a lot of donor fat, and wants a moderate augmentation. Sculptra may
take several sessions spaced several weeks apart. There is minimal
downtime associated with Sculptra buttock augmentation. I often plan
the degree of augmentation and verify with the patient how much Sculptra
I may need by doing a pre-run using Saline.  This will help me guide you as to how many vials you may need to fill in the defect.  If you were open to a surgical solution, then fat grafting may be helpful, and possibly an excision of the scar tissue if it is very hard and tethered to the underlying tissue.I would definitely consult with a board certified plastic surgeon who can guide you on your surgical options.  I would also look for a plastic surgeon who has a lot of experience in non-surgical options if you are looking to explore both options.

   Fat grafting to the buttocks can improve the cosmesis.  Kenneth Hughes, MDLos Angeles, CA....................

I am not sure if your problem is functional in addition to being a cosmetic problem.  Assuming this is a cosmetic problem only, you would likely benefit from fat injections into the area to fill it out.  I would avoid implants or other fillers.  A note on terminology:  Although it is a "cosmetic" problem, because it is a deformity from a previous trauma, anything done to correct it is rightly considered "reconstructive" and would be coverable by insurance.  At least that's how it typically works in the US, not sure about Canada.

In person consultation is an absolute necessity. There more than likely is a combination of issues your injury presents: there is a loss of volume, muscle as well as fat; There are most likely adhesive bands restricting skin expansion. All impact on the aesthetic outcome.  Understanding the defect we are attempting to correct is a must. Armed with that info, explaining the potential outcome becomes possible.  Once this is known, you and your Plastic Surgeon can determine if it's worth trying fat injections to that area.