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A common requirement is to “subvert” a token register that other macros may use. The requirement arises when you want to add something to a system token register (\output or \every*), but know that other macros use the token register, too. (A common requirement is to work on \everypar, but LaTeX changes \everypar at every touch and turn.)

The following technique, due to David Kastrup, does what you need, and allows an independent package to play the exact same game:

\let\mypkg@@everypar\everypar
\newtoks\mypkg@everypar
\mypkg@everypar\expandafter{\the\everypar}
\mypkg@@everypar{\mypkgs@ownstuff\the\mypkg@everypar}
\def\mypkgs@ownstuff{%
  <stuff to do at the start of the token register>%
}
\let\everypar\mypkg@everypar

As you can see, the package (mypkg)

• creates an alias for the existing “system” \everypar (which is frozen into any surrounding environment, which will carry on using the original);
• creates a token register to subvert \everypar and initialises it with the current contents of \everypar;
• sets the “old” \everypar to execute its own extra code, as well as the contents of its own token register;
• defines the macro for the extra code; and
• points the token \everypar at the new token register.

and away we go.

The form \mypkg@... is (sort of) blessed for LaTeX package internal names, which is why this example uses macros of that form.