Indeed, NOW I am trying to do the same myself.
So far, you can use intermol , this should work fine to convert LAMMPS data files to Gromacs files. Once you install intermol, and you ceate a path to the intermol converter, you can use a command like:
python2.7 $conv/convert.py --lmp_in topology.data --gromacs -v
CHECK the format of your data file, I still having problemst to convert it.
If you wish to create the psf file,
you would need VMD (google it), then open the tcl terminal and write :
topo readlammpsdata topology.data full
animate write psf topology.psf
The 1st line is for loading yur LAMMPS data file, if you are in the folder where
that files is located
2nd convert the data to psf CHARMM
Also, you could try this. In this paper, they provide a tood to conver
CHARMM topologies to gromacs here. Thus, you convert to psf, then to gro top.