This can only be done in the full desktop wallet. The mobile wallets and the Gulden-Lite wallet do not have this functionality.
In the main menu of the desktop, you can create a new account. Give the account a name and then under “import an account type” choose private key.
Then enter your private key in the input box.
After entering privatekey, the wallet will start synchronizing and an “imported legacy” account will be created where the Guldens will go. It is recommended to immediately send the Guldens from this account to one of the HD accounts because a legacy account is not under the protection of the recovery phrase.
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