Re: adding 'radiator zone' to a hydronic radiant floor system
I think your problem will be capacity. Even if you could reuse the radiators for hot water, you likely won't have enough BTU's to fully heat the house. Unless the existing system was oversized... which is common.
You need a HVAC contractor familiar with hydronic system to look at your installation and do some load calculations.
The likely outcome... when its really cold outside, you'll have to choose between heating the house to setpoint, or taking a hot shower.
It might be cheaper to just replace the existing boiler. You probably can downsize it substantially while your at it, since you have the other system in service.
1925 Two-Story Stucco Beaux Arts Neoclassical