Until now, CFRP was used to reinforce steel sections or to extend the service life in the event of fatigue. Now, the aim of the investigations is to find out whether CFRP can be used to convert an open, thin-walled Sigma steel profile into a closed one in certain areas. The tests were carried out on 16 samples, including reference beams (unreinforced) and three different reinforcement solutions as well as two different cross-sections. The experiments included two stages. Both stages used forked supports at each end of the beam, but stage II also blocked the possibility of cross-sectional warping. The laboratory tests used the ARAMIS system and the Artec Leo 3D scanner. The comparative analysis was carried out between vertical and horizontal displacements. In addition, global and local force-displacement diagrams were developed. The proposed reinforcement method can have a significant effect on increasing the load-bearing capacity of thin-walled steel beams with slender cross-sections, which tend to distort in the form of section opening.