What I mean is that using any programming language you’re comfortable with (Python, Perl, C++, Fortran, COBOL, etc.), write a utility that reads in the dump file, and for each atom, reads in the velocity vector and determines the speed of the particle. Then, using some sort of bins (where you choose how wide the bins are), you assign it to a bin representing speeds between values v1 and v2, where v1 < v < v2. You ten output the results to screen or to a file, for a single time point, or for an entire set.
Please take a look at texts like Allen and Tildesley, Frenkel and Smit, or Leach for more information on this. You may also want to take a look at the pizza.py package available from the LAMMPS website, which offers some tools to automate part of the data gathering process.
–AEI