How to Import an Ethereum Account into Polkadot.js Apps

In order to import a preexisting Ethereum-type account (e.g. created with Metamask) to Polkadot.js Apps, the following steps should be executed:

Open Polkadot.js Apps.
Navigate to the Settings tab.
In account options section, for in-browser account creation field, select from the drop-down menu Allow local in-browser account storage.
Press Save button.
Select Accounts page.
Click on Account button.
On the pop-up window, click on Mnemonic and select Private Key item.
Paste the private key of the account to be imported.
Check on "i have saved my mnemonic seed safely".
Press Next button.
On the next page, set an account name and a password
Press Next button and then Save.