If I understand you correctly, you’re saying that dynamic shadows are unsupported on your device and you want to be able to simplify the shadow manually? Well if its a character as you’re showing in your example image, then you cannot. Doesn’t matter if you change the shape, characters (and all skeletal meshes) are dynamic - which means they move around the environment - so any shadow you give them will be a dynamic shadow. If you’re wanting to add a shadow to a static object, then you could fake it with a custom decal material but why bother since your device should support static shadows.
Granted, I suppose you could make a fake material shadow appear below your character but this would only work if your ground is completely flat, or if your character is hovering in the air and you can place the shadow above the ground without it looking weird from the player’s view. Mostly though its not possible to do what you want without it looking really bad.
Simple unlit material on a plane beneath character’s feet. Only good for flat surfaces really (you can dynamically make it align to slopes and such). Needs animation if jumping, etc,
Slightly more complicated material that samples distance behind itself to make shadow appear a bit more accurately. (still has flat ground issue)