Most third-party wallets (e.g. Polkawallet, Trust Wallet, etc.) use a 12-word or 24-word mnemonic seed phrase to backup your account. However, if you created your Polkadot account in an older wallet version or a wallet that isn't supported anymore, it's possible that your private key is in the form of a raw seed instead of a mnemonic seed phrase.

To import this type of account into Polkadot-JS, you'll need to use the Polkadot-JS UI web wallet instead of the Polkadot browser extension.

First, extract your private key from your original wallet, and then follow the steps below to import it into Polkadot-JS. 

Before you start, please note: Exposing your private key online poses a security risk. You should only do this if you have no other options and if you are planning to send your funds to a new account with a new mnemonic seed phrase.

In order to import your private key, follow these steps:

1. Export your private key from your original wallet. You may have to check with the Customer Support of your wallet provider on how to do that. 
2. Once you have your private key, go to the Accounts page and click on "Add account"
3. Click on the word "Mnemonic" and select "Raw seed" from the drop-down menu that appears.
4. Delete the shown private key and enter yours instead.
   
   IMPORTANT: If your key doesn't start with 0x, add it manually in front of it. 

   
   

5. Check the box "I have saved my mnemonic seed safely" and click Next.
6. Follow the instructions and create a password.

Your account has now successfully been imported and you will see it listed on your accounts page.