How can I make an item in an entity's hand even if it's not a "Biped" model?

If it's a custom model, just put the item in there in modeling.  if you wanted like a sword or something in its hand, increase the damage.  

You can add handheld layer via custom code.

Could you elaborate @Evolving_Code

It's possible with Zombie models.

I'm not sure if it works with Skeletons, Villagers, Iron Golems etc.
but I think it works too, because:

• Villagers are able to hold an emerald or bread
• Skeletons are able to hold a bow
• Iron Golems are able to hold a rose

Btw, sry 4 my weird grammar lol.

Try the following steps:

1. Create a new Procedure ( Set item "Wand" in entites <entity> main-hand ).
2. Open the Triggers tab.
3. Use the trigger "On initial entity spawn" to run the procedure, you've createdin Step 1.

@Evolving_Code where do you put this script and what should it say?