The CO2 concentration (and other GHG concentrations) in openIFS is controlled via a couple of switches.
In the climate data directory, you find the files CO2CLIM and GCO2CLIM. The CO2CLIM and GCO2CLIM files however only define the distributions across latitude and height, not the absolute co2 concentrations. The real concentrations are controlled by various settings in the model.
The SRES-emmission scenarios (or better, their gas concentrations) are hard-coded in the model source code, and can be chosen with NSCEN in the "naerad" namelists. There is also code for the RCP (CMIP5) scenarios. It can be activated with
LGHGCMIP5=.true.,
GHGDATADIR="/path/to/ghgfiles/"
Then you need the input files
"PRE2005_MIDYR_CONC.txt"
"RCP3PD_MIDYR_CONC.txt"
"RCP45_MIDYR_CONC.txt"
"RCP6_MIDYR_CONC.txt"
"RCP85_MIDYR_CONC.txt"
located at GHGDATADIR. I successfully used the corresponding files from EC-EARTH with openIFS.