I have a question
Can I custom panel admin when app is already created?
In the documentation https://strapi.io/documentation/3.x.x/advanced/customize-admin.html#customization is written a project should be created in the dev mode with --dev flag, but I worked since 2 weeks on the project which I created without this flag, but now I want to add to admin panel one simple component and rebuild panel admin. Is this possible?
it's possible but the process is not easy.
What I would suggest is to create a dev app that has the same apis as your prod one. To do so you can follow step 1 and 2 from this issue.
Instead of recreating your apis just copy the api folder from your prod app into the dev one (if you made relations with the user model copy the ./plugins/users-permissions/models/User.settings.json file into the dev app, don't forget to restart your server).
Once you completed your modifications. Run npm run setup --plugins at the root of your dev application to make sure the build completes flawlessly.
At this point, you can reinstall Strapi globally npm install strapi@alpha -g.
Here you have two options, either copy your modifications into your prod application and run npm run setup --plugins at the root of the app or copy the generated build folder from the dev application into the other one.
Hope this helps.