You can use fs.readFile() and fs.writeFile() to read and edit the package.json file.
append-build-to-version.js:
var fs = require('fs');
fs.readFile('./package.json', (err, data) => {
    if (err) throw err;
    var packageJsonObj = JSON.parse(data);
    var build = 5; // get this from somewhere or increment the last char
    var versionNumber = packageJsonObj.version;
    packageJsonObj.version = `${versionNumber}-${build}`;
    packageJsonObj = JSON.stringify(packageJsonObj);
    fs.writeFile('./package.json', packageJsonObj, (err) => {
        if (err) throw err;
        console.log('The file has been saved!');
    });
});
package.json:
{
    "name": "teststackoverflow",
    "version": "1.0.0-5",
    "description": "",
    "main": "router.js",
    "dependencies": {},
    "devDependencies": {},
    "scripts": {
        "test": "echo \"Error: no test specified\" && exit 1"
    },
    "author": "",
    "license": "ISC"
}
More info on fs.readFile() and  fs.writeFile() can be found in the Node.js documentation.