Any ideas what can be added to pellets to make the filament flexible like Ninjaflex? (using Filastruder)

TPU and TPE filaments are fairly flexible.

So suggesting to try TPU or TPE pellets?

You want to buy plasticizers and stabilizers to add to your extrusion. There are a ton of different ones. You might want to download a plastic extrusion book.

Plenty of discussion here:

[www.soliforum.com]