If you can’t use Base.get_extension, you could always just define your own, i.e. in your main package have a function stub
#MyPackage/src/MyPackage.jl
module MyPackage
function get_foo_extension end
end
and then in your extension file you’d have
#MyPackage/ext/MyPackageFooExt.jl
module MyPackageFooExt
using MyPackage
MyPackage.get_foo_extension() = MyPackageFooExt
end
Users can then access the extension module with
MyPackage.get_foo_extension()