Is it possible to add a branch to existing fireplace blower?
Depending on the skill of the mason, local building codes, the configuration of the fireplace ducts and the cost I'm sure a new duct is feasible.   What you may want to consider is it worth the price and disruption to the household. Also would it make more economical sense to have a separate heating unit for the garage?   A gas fired ceiling hung 50000 BTU furnace can be purchased new for under $500. It might be worth it depending on the cost for the mason th reroute the fireplace duct.