Adding to a Monoflow Heating System

Hi All, I own a split level house with monoflow heating with those old space-eating radiators. The main line exits the hot water heater & runs along the top of the ceiling, with two splits that I assume feed the rooms on the 2nd floor. It then crosses into the family room where it wraps around via baseboard (not part of the original heating system, added on afterwards but before I purchased the house). Then goes under the crawl space & feeds the remaining rooms on the 1st floor before finally coming around back to the boiler.

I'd like to add heat to the room (it's a garage that's been partitioned off) adjacent to the family room. I've found the line that feeds the family room, and I assume is part of this heating circle. Question is, can I just cut a section of this line off, feed it to this "new room" (baseboard heating) and then connect the return back into the original flow so it continues onto the family room, etc... Or do I need some sort of tee, and if so where would they be placed?

Thanks in advance for your help.

pipe

You do not give pipe sizes or some other critcal information, but from what you do indicate, I am surprised it works satsifactorily everywhere. Without seeing the actual system, we cannot tell you whether your idea is sound or not, but my feeling is that you do not want to do it that way.