Use Burn-In Letterbox to burn a letterbox into a clip.
To access the Burn-In Letterbox menu, use:
Batch, then select a node from the Node bin.
Timeline, then add Timeline FX from the Effects ribbon (see Adding Timeline FX).
Timeline, then use Batch FX (see Creating Batch FX).
Tools, then select from the menu (See Accessing Tools).
This node accepts a front clip, and outputs a result.