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Here is a palette that does the slashing when you select a character and press the button:

CreatePalette[{Button["Slash it!", NotebookWrite[InputNotebook[],
     FromCharacterCode[
      Join[ToCharacterCode[
        ToString[NotebookRead[InputNotebook[]]]], {824}]]]]}];

It uses the unicode character for a combining solidus (824). This adds the slash to whatever character is printed in front of it.