In NinjaTrader 8, Heiken-Ashi can be selected as a bar type. To use Heiken-Ashi bars in NinjaTrader, open the Data Series setting (right-click on the chart background to find Data Series...), and set Type to Heiken Ashi.
Here's where you can find the Heiken-Ashi selection under Type, within the Data Series dialog box.
For NinjaTrader 7, an indicator or custom bar type is required to add the Heiken Ashi bars. Search the Ninjatrader Ecosystem for possible indicators. E.g., here's a link to one that incorporates an HMA smoothing function: https://ninjatraderecosystem.com/user-app-share-download/heiken-ashi-smoothed-v1-3a/