Blends of a copolyether-ester thermoplastic with different multifunctional (mono, di, and tri) amine-terminated polyethers have been carried out by reactive extrusion. The reaction with polyetheramine Jeffamine T-403 allows one to reduce drastically the viscosity of the blends while at the same time improving the Young's modulus of the matrix up to 30% and keeping elongation at break around 700% and stress at break around 20 MPa. The mechanism and the structures deeply scrutinized by NMR (in particular 2D DOSY), chromatography, and mass spectrometry evidenced the in situ formation of star-shaped-based structures. These complex structures are responsible of these changes in rheological and mechanical behaviors. This one-step approach represents an efficient and relevant way to enhance some mechanical properties while favoring the flow behavior of these copolyether-esters.