Hello Corensia,
I created your Korea Geoid tif file see first attachment. You can copy into the Agisoft geoids folder.
The prj is also attached ...
COMPD_CS["Korea 2000 / Central Belt 2010 + Korea National Geoid",
PROJCS["Korea 2000 / Central Belt 2010",
GEOGCS["Korea 2000",
DATUM["Geocentric datum of Korea",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","6737"]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],
AUTHORITY["EPSG","4737"]],
PROJECTION["Transverse_Mercator",AUTHORITY["EPSG","9807"]],
PARAMETER["latitude_of_origin",38],
PARAMETER["central_meridian",127],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",200000],
PARAMETER["false_northing",600000],
UNIT["metre",1,AUTHORITY["EPSG","9001"]],
AUTHORITY["EPSG","5186"]],
VERT_CS["Korea National Geoid",
VERT_DATUM["Korea National Geoid",2005],
UNIT["metre",1,AUTHORITY["EPSG","9001"]]]]
Also see geoid loaded on top of world Country map
Hope this works for you...