Some input files must be on the model grid - for example Initial Conditions files.
For interpolating between regular lat-lon grids, interpic can be used. It is provided in CESM code under ./components/cam/tools/interpic_new/.
For interpolating unstructured grids, ncremap ( https://nco.sourceforge.net/nco.html#ncremap ) works for some applications.
For example, to regrid a L93 IC files to L58:
ncremap --vrt_out=/glade/campaign/acom/MUSICA/grids/templates/template_L58.nc IC_L93.nc IC_58L.nc
Note that P0 is required to be in the template and input files, so you will have to add it to CESM3 files (e.g., ncks /glade/campaign/acom/MUSICA/grids/templates/P0.nc IC_L93.nc)