Yes, you can create a Google account with your Yahoo email address—or any other email address, for that matter—if you follow this link: https://accounts.google.com/NewAccount.
Just to make things clearer because your question mixes some issues that might lead to confusion:
If you want to sign up for Google Plus with a Yahoo email address and you already have a Google account under that address, you cannot sign up for Google Plus separately. It will all be linked in your Google account.
If you use only Google services that do not require a Google account—such as Search, Maps, Translate—and you have not created one yet, you will be forced to create a Google account upon signing up for Google Plus. After all, it is linked to your identity and therefore you need to create an account (I know I am stating the obvious, but one should never assume any prior knowledge).