You can't really decouple the password from the username and signature -- they're a set, and you generally have to provide all three whenever you make an API call.
However, you can do a limited amount by passing the Subject
field instead of the API username, password, and signature. In this case, the subject would be the email address of your PayPal account. However, without the API username/password/signature, you're going to be limited to running just a few API calls -- SetExpressCheckout, GetExpressCheckoutDetails, DoExpressCheckoutPayment, and GetTransactionDetails. You're also going to be limited to running Sale transactions -- you can't run Authorizations, and you can't create recurring payments.