With Due Web App:
After signing up with Due, you will be prompted to either created a Vault or connect an external wallet. Select 'Connect your wallet'.
Select your preferred wallet provider.
Approve (or "sign") access via your external wallet.
Click 'Save' and you're good to go! Your external wallet is now linked up.
With Superlite:
Right now, you cannot connect an external wallet directly in the Superlite app. With Superlite, a self-custodial wallet is created for you during onboarding, which will serve as the Vault for your account.
If you wish to connect an external wallet in addition to your Vault, you can do so by logging into the Due Web App from your desktop, and logging in using your Superlite credentials.
Follow the steps above to link your external wallet.
To switch between Wallets thereafter, just head to [Settings] → [Wallets]