There are numerous ways this can be done. Taco makes an add-on relay box that can control multiple circulators and it OR's the call for heat to the boiler. You may need to add on more stuff, though, as the typical water temp for the baseboard is likely way too hot to run in in-floor radiant. So, some sort of mixing or tempering valve would need to be installed as well. With an older boiler, the run time on the radiant zones my short-cycle unless you add some buffer. Would need more info on what the actual loads are in the various zones to come up with the most ecconomical solution. Hopefully, you had someone run the numbers so you have the pex runs at the right spacing and density for the size of the heating area.