It's not a simple branch and flow job!
As opposed to the radiator, in-floor radiant needs to be a lot lower in temp, so you'd need a mixing valve to adjust the temp. Then, you'll probably want diffuser plates (usually aluminum) to spread the heat across the area. Then, you need to insulate beneath the pex. Depending on the current circulators and flow, you may need a new zone which could include a circulator.
A common operating temp for radiators is 180-degree water...you won't want more than probably 120 or so in the floor, maybe less, depending on design and flow. You normally don't want the floor temp higher than around 80-degrees. These numbers are not specific, but generalizations.