As with a processor port, usually the first step is to publicize your
intention and make sure that nobody else is already doing this. The
second step is to make sure that you have the necessary tools, in
particular some way of running an executable on the target board. It
is possible that gdb already provides support for that board.
Alternatively it may be necessary to add the necessary support to gdb,
or to use some other means of running programs.
Assuming a working toolchain, the work involved on the eCos side is to
implement a new platform HAL package. This involves issues such as
board initialization, diagnostics, possibly gdb support, and linker
scripts. There are existing platform HAL packages for the supported
boards, and any of these can be used as starting points.