How the stepness can be computed at NDBC from site with only dominant wave data and windwave data
The methodology to calculate wave steepness used by NDBC is explained in: http://www.ndbc.noaa.gov/algor.shtml

Steepness is defined as
$\xi=\frac{2\pi H_s}{gT_z^2}$ where $H_s$ is the significant wave height and $T_z$ is the average wave period.

From your dataset, you have significant wave height and average wave period so you should be fine.