First, be sure to logout of any Yahoo! account before you start this process.
Next, start the standard import league process. When given the option, click the "Add New Yahoo! Account" button, input your Yahoo! username and password that you wish to import from, and attempt to authorize access.
If this does not work for you, the other option is to email us your Yahoo! username and password for your other accounts and we can attempt to import your league on your behalf.