I happened to have wrote a similar one:
You may wrap the build step in the build.jl and then ]dev the package locally or register the subfolder after Julia v1.5. If the original package is quite stable. I’d suggest to create an independent Julia package and create a build script on Yggdrasil just like @jstrube said. By doing so, you can have more control of bugfixes/adding new features specific to Julia.