As a temporary workaround, if you find that ne already has support for a language with somewhat similar syntax and keywords to Julia, you can copy the relevant $ne-global-dir/syntax/whatev.jsf file to your ~/.ne/syntax directory, renaming it in the process to julia.jsf. For example, you might try:
cp /usr/local/share/ne/syntax/python.jsf ~/.ne/syntax/julia.jsf
(though I see that doesn't catch `#= multiline
Then whenever you open a .jl file, run `^k sy julia` first thing to enable the syntax highlighting.
I don't think there's currently a way to configure ne automatically run that `^k sy julia` for you upon opening a .jl file.