From the NI site, FAQ section on applying for DPO:
"[B]Is watchkeeping time accrued on board any DP equipped vessel eligible to gain a DP certificate?[/B]
"Only watchkeeping time gained working as a DPO on board vessels officially classed as DP by one of the classification societies will be counted towards your required time."
Sending in a copy of your vessel’s DP Certificate of Classification should do it.