Can I put uncooked chicken marinade back on chicken after it is almost finished on the BBQ or roasted?
If it's the same marinade you marinated the chicken in, I would be very careful. If it's not cooked to a high enough temperature, it is not safe to eat, as it has been in contact with raw chicken. It would probably be safe, but I would just mix up a new similar batch and use that to finish off the chicken. It's not worth risking salmonella to save the cost of making a little more marinade.
It's unwise to do that as it could carry salmonella or other foodborne illnesses from the uncooked chicken. Although mopping some on in the last minutes of cooking will be ok most of the time there's always a possibility some will not get cooked enough.
The simple answer to this is for you to cook your marinade for a bit before you use it as a baste. All you need to do is heat it to 160F (70C) for a minute and it will be safe. It will impact the flavor a bit but I'd take the trade-off any day.