http://www.ctan.org/tex-archive/info/symbols/comprehensive/symbols-letter.pdf suggests that \slashd does something like that not sure if it's supported or if you need a package (search for slash in that pdf)
Also check out p.100:
"The slashed package, although originally designed for producing Feynman
facilitates the production of arbitrary overlapped symbols. The default behavior
with \=". For example, \slashed{D} produces \ = D". However, the \declareslashed
exibility to specify the mathematical context of the composite character
will be discussed in Section 7.4), the overlapping symbol, horizontal symbolrelative
units, and the character to be overlapped. Consider, for example,
moment (\ - I"). This can be declared as follows:
\newcommand{\rqm}{{%
\declareslashed{}{\text{-}}{0.04}{0}{I}\slashed{I}}}"