Following thanks to Gianguido BALDINOZZI (baldinozzi@cps.ecp.fr)
"If you need to enter missing space groups in xnd, just use sginfo -allxyz Pm-3m then copy and paste the first block of symmetry ops (24) in xnd adding the correspondent headers in the .k input file ; for example in this case it should be PM3M C 1 1 24 and the corresponding 24 sym ops in free format xnd will rewrite in a more compact way the block in the .new file after you run it."