if you have grads, after open it, you can use this command to convert netcdf and merge them,

lats4d -v -q -i 89.MScSim/ATM_P.mon.ctl -o first.nc
lats4d -v -q -i 90.MScSim/ATM_P.mon.ctl -o second.nc
cdo mergetime first.nc second.nc result.nc

There is a cdo command also, however it doesn't work with some of the Regcm outputs because of pdef definition,

you can try it also
cdo -f nc import_binary 89.MScSim/ATM_P.mon.ctl first.nc
cdo -f nc import_binary 90.MScSim/ATM_P.mon.ctl second.nc

Best Wishes
Hamza Altınsoy