Create Mock object that doesn't have setters

I'm a newbie using NUnit&Moq and I'm trying to create a complex mock object to use in my unitTest project. The main problem that I have is that the object I'm mocking doesn't have defined set in their properties. As result I can't compile the following code.

m_objectMock.Object.Id = "myMockId";

Is there another way to achieve this assignation without adding setters to the class I want to test?.