Pack then import CSS from a local library (Angular)

Try this based on npm link

File Structure
/projects/my-scss
/projects/my-existingproject

Create a new project
cd /projects
mkdir my-scss
cd my-scss

Initialize the project and answer the prompts
npm init

Dump a SCSS file in there

// _base.scss
$font-stack:    Helvetica, sans-serif;
$primary-color: #333;
body {
  font: 100% $font-stack;
  color: $primary-color;
}

Navigate to your existing project
cd ../my-existingproject
npm link ../my-scss

Verify the my-scss folder exists in the node_modules of your existing project.

Then suppose you want to get all the *.scss files in your my-scss project and put them in the /wwwroot/scss folder of my-existingproject. Gulpfile.js within my-existingproject would look something like

const gulp = require('gulp');
const { src, dest } = require('gulp');
const merge = require('merge-stream'); 

var deps = {
    "my-scss": {
        "**/*.scss": ""
    }
};

function scripts() {

    var streams = [];

    for (var prop in deps) {
        console.log("Prepping Scripts for: " + prop);
        for (var itemProp in deps[prop]) {
            streams.push(src("node_modules/" + prop + "/" + itemProp)
                .pipe(dest("wwwroot/scss/" + prop + "/" + deps[prop][itemProp])));
        }
    }

    return merge(streams);
}

exports.scripts = scripts;
exports.default = scripts;

Then providing you have installed gulp and all required gulp-modules run 'gulp' from the project directory command-line. This will run the default task.