There are many, many ways to “smoothly” join potentials together. hybrid/overlay does not do that, it justs adds the potentials together everywhere. A specific example of splicing in LAMMPS is Tersoff with ZBL:
http://lammps.sandia.gov/doc/pair_tersoff_zbl.html
As Axel said, you could follow a similar procedure to construct your own two-body potentials and force functions, and provide them to LAMMPS using the table pair style.
Aidan