Map Values during installation

Hello,
      We have a Map in which a couple of  node in the destination will have values according to Environment we deploy
      the Solution into.
      For e.g.: when we deploy into Production, the two nodes UserName and Password must have  the production values. Similarly
      when we deploy into Test, the node must take the Test environment values.
      Ideally,we would like the MSI to prompt for these values at the time of installation, but then how would the map get this values.
      Any suggestion would be greatly appreciated.
      Thanks
regards,
BM