Can I Connect the Underfloor Heating to the Radiators Circuit?

Only if the area isn't greater than 15m2 and the Return Water Temperature Limit valve is being used. If the UFH system is connected to the radiators it can't be controlled individually.

For all rooms greater than 15m2 you need separate primary flow and return pipes from the boiler or you can tee off the main central heating pipes (before the first radiator).