Q: How can I add add repositories from arch into manjaro?
I would like to install a few applications from arch repository without having to compile. Because I am a Windows user who new to arch base distros, I would like to get my feets wet before trying to compile my own package.
Thanks.
Wise decision 
When you choose to build packages from AUR you need to know that every time you update your Manjaro system - there is a chance that your AUR packages will need rebuild and therefore - when an AUR package acts weird after an update - rebuilding said package is often the solution.
You cannot - and most importantly you should not as it will cause irreparable damage to your system files.
Manjaro repos is drawn from Arch and synced several times a day from Arch stable.
Except from some obvious packages like kernels, kernel modules, nvidia drivers, a few Arch specific packages (e.g. arch-install-scripts) and Manjaro specific applications - there is nothing in the Arch repo which is not available in Manjaro repo.
Hello,
If you are referring to this
https://wiki.archlinux.org/index.php/unofficial_user_repositories
then reading trough might help. Just proceed with caution.
Howdy!
By installing yay sudo pacman -S yay , you will have access to the AUR via the terminal!
Yay’s usage is the same as pacman!
Have a wonderful day!