Software for generating a lattice from a unit cell for molecular crystals?
In addition to the presentations about CCDC Mercury and Vesta by @andselisk here and later mine about Avogadro could be extended.  As described earlier, Avogadro's capability to read .cif files about crystallographic models allows you to extend these  via the slab builder to any extent you need (GUI entry Crystallography -> Build -> Slab) and to export them either as normal .xyz file, or (via the extension tab, template assisted) in one of multiple formats recognized by quantum chemical programs (e.g., Gaussian, GAMESS-UK, MOPAC).

It equally is possible to start from scratch with a cubic unit cell of 3 Å.  Subsequently, you would alter the lattice parameters ($a, b, c; \alpha, \beta, \gamma$) and enter atoms one, after the other, either in cartesian coordinates (either Angstrom, Bohr, nm or pm), or in fractional atomic coordinates as typically in .cif files. You may wrap the atoms into the cell, and of course impose one of the space group symmetries to fill your cell, too.  You may symmetrize your constructed unit cell and reset (within tolerances you determine) into a standard setting, too. Equally than just reading a .cif,.pdb, or .xyz, these constructed unit cells may be extended with the slab builder, and equally may be exported in one of the computer chemistry formats mentioned earlier, too.