Hi Kiran-
I'm trying to do the same thing... a book I have found helpful is 'Computer Simulations of Dislocations' from Bulatov and Cai. Also, a paper by Chang, Cai, Bulatov, and Yip, Computational Materials Science 23 (2002) 111-115.
Relevant lammps commands are what you would expect - lattice: particularly the orient and spacing commands, region, group, delete_atoms, displace_atoms, etc...
For periodic bcs, you will need a dislocation dipole, so the sum of burgers vectors is zero; for a single dislocation you can have periodic in some directions and free surfaces in others. Edge dislocations are basically made by removing or adding a plane of atoms; screw by shifting atoms. Then other atoms should be put into place according to linear elastic displacement theory (this is the part I am working on)...
Hope that helps a little. If anyone else has suggestions, I would like to hear them too; I'm just getting started on this project.
Erin