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fasta2nexus with seqkit and paup (when you need to convert a lot of files):
for k in *.fas ; do seqkit fx2tab $k > infile; paup -n toNexus.nex; mv outfile $k.nex; done

toNexus.nex is:
#NEXUS
Begin paup;
toNEXUS format=text fromfile=infile tofile=outfile replace=yes;
End;