To answer my own question here, there does not seems a way to do that. To manually add a target, here are the three parts that need to change. I am using FreeBSD as an example:

1. add the target name "x86_64-unknown-freebsd" to package.json, under napi.triples.additional
2. add the target folder freebsd-x64 under npm. create package.json there.
3. This is a little bit complex: add the building process to .github/workflow/CI.yml. Since there are multiple moving parts, it is better to create a clean dummy project and later copy part of the generated CI.yml file in.

Hope it will help.