Paul,
I have no idea what a DAT format for volumes would be. The matlab binary format is .MAT. If that is the case, then that file may contain any matlab variable and not just a volume. You will need to load this file into matlab, see what is in it: whos, locate the volume and its header structure or variables and then try to put it in AFNI format. The last step can be done with AFNI's matlab library: [
afni.nimh.nih.gov]
cheers,
ziad