If you want this because you are concerned about the security, the current way it works is pretty secure.
Even if the merchant redirects you, it opens a new browser window/tab of Paypal's own web page. You can check the identity with https indicator in you browser bar. You enter your password there, logging in to Paypal yourself. Merchant redirecting you doesn't affect the security in any way.
Nope, there isn't a way to do that.
If you want this because you are concerned about the security, the current way it works is pretty secure.
Even if the merchant redirects you, it opens a new browser window/tab of Paypal's own web page. You can check the identity with https indicator in you browser bar. You enter your password there, logging in to Paypal yourself. Merchant redirecting you doesn't affect the security in any way.