Hi Zhen,
from MicroStation XM Edition it's possible (and recommended) to use ICO or BMP format to define custom icons. Original icon rsc format has been treated as obsolete now. Consequently icon editor was deprecated, so I think now there is no tool how to create or import raster data into rsc format.
I recommend Jon's article how to use Windows icons in MDL projects.
But as was discussed in another thread, the most simple (and preferred) way is to use dgnlib, bucause customization tool offers GUI to import different raster formats (rsc, ico...) into dgnlib and to define tools manually without programming.
With regards,
Jan