it’s possible to use waste plastic (specially polyethylene PET, polypropylene PP or polyester ) but must be cut at proper size (1,8-2 cm to 4cm length) and must be enough thin to be flexible.
But every fiber is designed to increase flexural ability, crack bridging and other tensile strength of concrete. Elastic module and compressive strength are very less influenced by adding of fibers. Elastic module is more strictly connected to compressive strength and type of crystalline matrix that concrete has developed during hydration process.
So so in my experience plastic Waste fibers shouldn’t be the best solution to increase these parameters.