How do we establish traceability for an in-house reference standard?

Traceability (VIM 2.41) needs an unbroken, documented chain of calibrations, each with stated uncertainty, ending at the SI. Per ILAC-P10, acceptable sources are:

1. An NMI (NIST, PTB, NPL…) whose relevant CMCs are in the BIPM KCDB;
2. A calibration laboratory accredited to ISO/IEC 17025 by an ILAC-MRA signatory, with the service on its accredited scope — look for the accreditation symbol and certificate number on the cert.

A factory 'certificate of conformance' without accredited status, uncertainties, and standard identification does not establish traceability. So: send the DMM for an accredited calibration covering the functions/ranges you use, at intervals you justify, and keep the chain of certificates. Then your internal calibrations against it are traceable — provided you evaluate and state your own uncertainty on top.