I think thre following prj should do the tricK:
COMPD_CS["WGS 84 / UTM zone 17N + EGM2008 geoid height",
PROJCS["WGS 84 / UTM zone 17N",
GEOGCS["WGS 84",
DATUM["World Geodetic System 1984",
SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],
AUTHORITY["EPSG","4326"]],
PROJECTION["Transverse_Mercator",AUTHORITY["EPSG","9807"]],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",-81],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",500000],
PARAMETER["false_northing",0],
UNIT["metre",1,AUTHORITY["EPSG","9001"]],
AUTHORITY["EPSG","32617"]],
VERT_CS["EGM2008 geoid height",
VERT_DATUM["EGM2008 geoid",2005,AUTHORITY["EPSG","1027"]],
UNIT["metre",1,AUTHORITY["EPSG","9001"]],
AUTHORITY["EPSG","3855"]]]
Hope this helps,