Is there a cheat that turns a ghost into a living solid sim?

Removing the death trait (found in the simology panel and tells how this sim died) won't help, so like others have said, either make an ambrosia or use the wishing well from Romantic Garden stuff.
But you can use cheats to revive the ghost faster.

Ambrosia method:

• Enter this cheat in build mode:
  

  bb.showhiddenobjects

  
  
• And search for angelfish, death flower, and potion of youth. You'll have to search them in the search bar.
  
• Find a sim who you want to cook it and max the cooking skills by using these cheats:
  

  stats.set_skill_level major_homestylecooking 10
  stats.set_skill_level major_gourmetcooking 10

  
  
• Put the ingredients into either a fridge or that sim's inventory and have them make the ambrosia.
  
• After it's done, have ghost eat it and they will be resurrected.

Wishing well method (Romantic Garden stuff required):

• Buy a wishing well and enter this cheat:
  

  testingcheats on

  
  
• Now by shift-clicking on it you can change its outcome. You'll want either positive or overwhelmingly positive outcome.
  
• Have your ghost wish for life.
  
• If you pick positive outcome, your sim will either receive ingredients to ambrosia or ready-made ambrosia. Either do it again and again to get a ready-made ambrosia or have a sim with maxed cooking skill make one with the ingredients.
  
• If you pick overwhelmingly positive outcome, your sim will be resurrected and get immunity to unnatural death like unlucky sims in The Sims 3 (That means that sim can die, but they'll be brought back to life unless sim dies by old age).