Input time series into EnergyPlus from csv or txt files

Is it possible to import (txt, csv) files into EnergyPlus for running time-series simulations (changing a variable with every time step). For example, I want to model a concrete core heating/cooling system and the supply temperature and mass flow should be set to measured values (time step 10 minutes).

Or do I have to write them by hand into schedules?

It sounds like you need Schedule:File objects. These refer to a CSV file and column number which contains your input variables.

In the situation you describe though, I think you may also need to use EMS, since I'm not sure mass flow rate can be controlled by schedule directly (though there is a Pump Flow Rate Schedule Name field for the ZoneHVAC:LowTemperatureRadiant:ConstantFlow object).