How do you add on to an existing claim , reassess for a greater amount.

The answer is yes, if they assess a higher amount they will back date to the submission date.

So my existing VAC pension is under review for a possible greater amount, which will take a minimum of 16-24 weeks. If my existing injury is re-assessed at a greater amount...lets say 5%, and it takes 4 months to process my claim, does VAC provide back dating of the money i`m now entitled to? i.e. I submitted May 30/17, if reviewed in my favor Oct 1/17 , will I receive 4 months at the missing 5% or will I just start receiving money monthly at the new overall greater %, a jump in pay of 5%?

It's not a reassessment of an existing claim. That is only if the actual claimed condition worsens. What you are looking at, is a consequential claim. It a brand new claimis the same process as before, but instead of looking only for how your service caused the disability, they look at how the existing claimed condition could cause or aggravate the new condition. Example " Veteran A loses part of his left leg to an IED. He obviously gets a 5/5 pension. But say two years later, he develops arthritis in the opposite leg from only weight bearing on the remaining leg. That would be considered a consequential disability, that otherwise would not have presented itself, or worsened, if not for the amputation.

I looked the form up on the VAC site, but it asks questions like "can you drive a car" etc. but it doesn`t really seem like the format lends itself to explaining that this is a reassessment for an existing claim.

I would use the regular form and write clearly 'consequential to ... '

I apologize in advance if my question(s) are redundant/repetitive . I was assessed in 2006 at 40% but it has come to light that one of my other injuries is claimable, what is the process. It is directly related to my original claim but I don`t even remember what forms I filled out back then. Do I open a whole new claim, what form is that? Or is there a special  form you fill out to add to your original assessment.