You possibly could tap into the system. It depends if you have excess
capacity. How many hours a day does it run at your coldest time of
year? I tried tapping into my system, Its large enough as it only runs
8 hrs at -15. But when the sun is out I dont call for heat and the
basement gets cold again. For that reason im installing a secondary
furnace, only to heat it when i want to.
If it was my house and you were at your coldest outside temp and it
didnt run 18 hrs a day Id start cutting , its easy to put in ducts, and
easy to seal them off. Yes you will unbalance the upstairs, and take
heat away , First you need a basement return. and you could do the
supply easily by using insulated flex duct to get the heat where you
want it.
Perhaps you could get a bid to see what a pro wants and recomends. For
me it was DIY Sawsall, and flex.