Dear sumit,
the source, where your elevation data comes from, doesn't matter. You have to convert your data into a dfs2 - File with elevations.
MIKE21 offers a possibility to convert ASCII Grids (ESRI ASCII GRIDS) into MIKE21 - dfs2 - Files with the MIKEZERO - Toolbox.
Otherwise you can create DEM directly in MIKE with the Bathymetry - Editor - please refer to the MIKEZERO Online Help.
Bests Christian