It can be a little tricky to get the file imported because you’ll need to make sure Quicken associates it with the correct account. You may need to change the name of the bank on your account before you import the file. The easiest way is to create a new account when you import the file as shown in the image. You can then go in and edit the name of the account, but you’ll need to leave the name of the bank set to Washington Mutual.
Another possible way to work around the limitation is to redirect all traffic to Intuit to your local machine and return the proper codes to authorize the transaction regardless of which bank a QFX file comes from. Ideally someone could setup their own server to act like Intuit’s server and return affirmitive responses to all requests.