First, if you haven’t already done so, check whether the distribution from the defined-benefit pension plan is rollover-eligible. While you know that law, many participants have mistaken assumptions.
Also, the participant should check whether the governmental 457(b) plan allows Roth contributions to designated Roth accounts, and whether the plan allows an in-plan Roth rollover. Not all plans provide this.
Even if tax law might permit this participant’s desired move to Roth treatment in one transaction, a participant depends on the receiving plan’s recordkeeper’s practical ability and willingness to make the records that support the desired tax treatment. One suspects a recordkeeper might prefer the two-step. Anyhow, the participant should ask the recordkeeper what to do so the processing would get the desired result.