It is
possible, but is it desirable? If the dataset is of any size, the resulting text file will be huge. A 64x64x30 volume has 122,880 voxels, so you would get a 1D file with 122,880 lines of text.
If you insist, you can try something like
3dmaskdump -noijk FILE.hdr > FILE.1D
and see if you like the result. 3dmaskdump can also give you voxel coordinates, or just give part of the dataset -- see the output of
3dmaskdump -help
for details.