Ok here it goes I have added a 2 story addition to my home. I have radiators threw out the house. I have 2 main lines coming of the boiler one feed the other a return. then each radiator extend of the main lines. I want to add base board heat to the addition can I just create 1 loop for both floors of base board heat and connect it right back to the mains like the other radiators in the house. also if I can is it possible to use pex to go from the main to the first baseboard and from the last baseboard back to the main. idk if its important but it's roughly 240 sq per floor how many feet of baseboard would you think i need for each floor thank you in advance for any help because I'm clues less about heating system
What you are proposing sounds doable.
You should be able to tie into an existing boiler. It is best to keep baseboard and radiators on separate zones. Your contractor will have to verify capacity.
Pex with an O2 barrier is suitable for heating systems.
The amount of baseboard needed will be based on the heat loss of your addition. Calculations will need to be done.
There are many variables when tying into an existing system. A good contractor will help you work through them.