This requires no extra commands at all.. The head spawners are in the creative inventory and if you kill a zombie, skelleton, or creeper it will drop a head spawner for that mob.
First of all, to use any mod/mods you will need Blocklauncher
or Blocklauncher pro. You can find it on the app store (for kindle and android only).
Step 1 : Download any mod/mods of your choice.
Step 2 : Open Blocklauncher(free/pro).
Step 3 : Click the wrench icon near the top of your screen.
Step 4 : Click the manage ModPE scripts option.
Step 5 : Click the import button.
Step 6 : Click the Local storage button.
Step 7 : Find where the mods that you downloaded went (usually under downloads).
Step 8 : Click the mod that you wish to install.
Step 9 : Enjoy the mod/mods that you installed.