Overview
Both the rostoolchain and rosplatform scripts can be used to create/save/delete your own custom toolchains and platform configurations in a user-defined library.
Location
By default, these libraries are located at $ROS_HOME/eros/toolchains and $ROS_HOME/eros/platforms (i.e. if you are on linux, ~/.ros/eros/toolchains). However, they can be redirected when calling rostoolchain or rosplatform by using the --dir option.
They can be more permanently redirected by editing the eros configuration file ($ROS_HOME/eros/eros.cfg).
Editing
If you need to modify these, you can freely modify them by hand without disturbing anything.
Submitting
Feel free to contact the developers if you wish to add your custom user-defined modules to the official eros_toolchains or eros_platforms library. The same also applies if you think any pre-defined configurations need modification.
Probably the best place to do so is via the eros issue tracker.