If you want meaningful help, you need to provide more detailed information and context.
Please see Please Read This First: Guidelines and Suggestions for posting LAMMPS questions for some explanations of how things work here.
For starters you need to say which “flavor” of DPD you want to use. LAMMPS has at least 15 different ones.
In general, information about the inner workings of LAMMPS are in: 4. Information for Developers — LAMMPS documentation and 3. Modifying & extending LAMMPS — LAMMPS documentation