add additional subcommand for the "chan" tcl command?

Hi,
  I would like to add "chan exists str" just to signal a boolean return on exists and I want to do this
  *without* to touch the tcl distribution.
  there is already a feature like "::tcl:mathfunc::????" to add an additional function to "expr" and I
  ask me now it is possible to do such a kind of "addition" to the "chan" command as well ?
mfg ao