While you can't directly link an unsupported bank account, you can still add its details manually. Typically, Plenty members will update the balances for their manually tracked accounts 1-2 times a month. 

Just follow these instructions if you'd like to manually add a bank to your Plenty profile: 

1. From your Dashboard, click on the "+" to the right of the "Accounts" section
2. Select "Add an account manually"
3. Pick the relevant account type
4. Fill out the fields for account name, current balance, and any other relevant information.
5. Hit "Continue," choose the sharing settings for the account, and click "Looks good"