Laurent Duveau blogged about a solution for inheriting from an Implicit Style.
Since Silverlight 4 you can use Implicit Styles:
http://weblogs.asp.net/lduveau/archive/2009/11/18/silverlight-4-implicit-styling.aspx
Basically it is just a style defined in resource without a key. As you don’t
have a key you may wonder how you could inherit (BasedOn) from an
implicit style?
Surprisingly lots of people are not aware of this, so here is how you can
inherit from an implicit style: