You can't directly add an existing Facebook account to your current profile. Each Facebook account is unique and independent. However, you can switch between accounts you already have. Here's how:
- Open the Facebook app or website.
- Click on your profile picture in the top right corner.
- Select "Log Out" from the dropdown menu.
- Enter the email address and password for the account you want to switch to.
If you have multiple accounts logged in on the same device, you can also switch between them without logging out.
- On the Facebook app: Tap your profile picture, then tap "See All" and select the account you want to use.
- On the Facebook website: Click the down arrow next to your profile picture, then select the account you want to use.
Remember, each Facebook account is separate and you can't merge them. You can only switch between them as needed.