How can I show this similarity on my resume?
If you say both tools (WPF and Silverlight) use basically the same technology, you could try listing the underlying technology in your resume instead of the derivate product (WPF or SL).
To give an example, lets say both WPF and Silverlight use techologies
Bar. Then instead of saying you are expert in Silverlight you could say:
Experience with Foo, CoolXML, and Bar (Used in tools like WPF or Silverlight)
This way you will not be lying or falsifying your resume by saying you know WPF (which you say you don't know currently) but still the recruiters will see the word they want in your CV.
Just be sure that if you are ever asked answer truthfully and don't say you know WPF. This could come back at you in a negative way. In the meantime, if those tools are indeed quite similar I suggest you start learning about WPF. Surely you will get it faster as you already know the underlying tech they use.