[lammps-users] XYZ format and box length


I was wondering If it is possible to add simulation box lengths (lx,ly,lz) in XYZ format of the dump file?

No. That would change the format in an incompatible way. It would break programs reading it.
Just use a different dump style that includes the box information. Or create a xyz/box dump style that has the change you want in the way you want.
This is the good thing about LAMMPS being Open Source. You can actually do such things and make the software work the way you want.
For the general distribution it is not worth the effort since there are already better and more flexible formats that support exporting that information.