Is it possible to access for GLSL shader in Unreal engine with C++ ?
No, unreal has a cross compiler, it converts HLSL code to GLSL automatically when use that platform. Unreal uses a special format called .usf or .ush (unreal shader file/header) where you can write your HLSL code. You can check in the source code how some post process shaders are made. I think the easiest example there is the PostProcessPassThrough.usf, check the MainVS and look for its c++ implementation, all PP are more and less the same thing, they all use RDG since 4.26.