Thank you for using our services.
I can suggest that you do the following thing. Sitefinity's login controls are not embedded into the dlls. They can be found in ~/Sitefinity/UserControls/Login/.
What you can do is to modify the LoginViewControl.ascx
control to display whatever you want. I have attached a sample user control that extends the LoginViewControl, you can download it and see how it is done.
I hope that this is what you need to do. If you have more questions, please feel free to contact us.
the Telerik team