Hi Alessio,
The XyGrib gribserver provides global models and large area regional models at medium resolution. We do not include local/regional models at high resolution due to the huge volume needed by such models such as cosmo.
We do however provide pre-cut and packaged Cosmo D2 grib files on our sister site at
openskiron.org/en/cosmo-gribs. These are targeted to the sailing community.
It is possible to concatenate (join) the files using the appropriate command line commands. Just Google "how to concatenate binary files in windows or linux"
in Windows cmd terminal try
COPY /B File1.grb+ File2.grb+File3grb file_joined.grb
In Linux
cat file1.grb file2.grb file3.grb > file_joined.grb