Adding RFH to Old Rad.Syst

Newt Loken

Existing condition; High temp, single pipe classic radiator system on a boiler. Desire; Add some medium temp,low mass, radiant floor heating zones.
Primary question; Can it be done without replumbing Primary/secondary.
If Pex is used we may mix down the temp. We could use something like Wirsbos multicore tubing with same temperature radiator water (160). Anybody doing that?
Thanks for any leftover grey matter.