Access another project over an xmlns reference

I am using a custom calendar control made by some开发者_运维知识库one else (under the GNU-GPL, of course) that I loaded into my solution. So how can I create a xmlns reference to it to use the calendar control? Also, they have different namespaces.

From XAML Namespaces and Namespace Mapping:

xmlns:custom="clr-namespace:SDKSample;assembly=SDKSampleLibrary"