Yes, if you just need to setup a few users then you run a script locally to add the claims. Just follow the steps:
1. Create a NodeJS project
npm init -y
//Add Firebase Admin
npm install firebase-admin
2. Write this code in a JS file (addClaims.js):
const admin = require("firebase-admin")
admin.initializeApp({
credential: admin.credential.cert("./serviceAccountKey.json"),
databaseURL: "https://<project-id>.firebaseio.com/"
})
const uid = "user_uid"
return admin
.auth()
.setCustomUserClaims(uid, { admin: true })
.then(() => {
// The new custom claims will propagate to the user's ID token the
// next time a new one is issued.
console.log(`Admin claim added to ${uid}`)
});
3. Run the script
node addClaims.js
You can also write a function that takes an array of UIDs as parameter and adds claims to all of them using a loop. To read more about service account and initializing the Admin SDK, check this documentation.