Hello, I have added the .lef file of my standard cell to the SRC folder, but when I run openlane I get that the module does not belong to my design, does anyone know how I can solve this? I want to highlight that I don't want to replace standard cell, I want to use my own cells in my own design

Is that a macro cell or standard cell? For macro you need to use the blackbox declarations for Yosys to recognize it

does a modified standard cell count as a macro?

it can be used as macro, but uf you want it to be a standard cell then flow is much harder and requires modification to OpenLane or PDK