Construction and enlargement of traversable wormholes from Schwarzschild   black holes
  Analytic solutions are presented which describe the construction of a traversable wormhole from a Schwarzschild black hole, and the enlargement of such a wormhole, in Einstein gravity. The matter model is pure radiation which may have negative energy density (phantom or ghost radiation) and the idealization of impulsive radiation (infinitesimally thin null shells) is employed. 
Hiroko Koyama, Sean A. Hayward