To modify behaviour of MLJ you will need to clone the relevant component package (e.g., MLJBase.jl) - or a fork thereof - and modify your local julia environment to use your local clone in place of the official release. For example, you might proceed something like this:
using Pkg Pkg.activate("my_MLJ_enf", shared=true) Pkg.develop("path/to/my/local/MLJBase")
To test your local clone, do
For more on package management, see here.