It is unfortunately not currently possible to add fonts from an add-on since ReShade clears and rebuilds the font atlas from scratch whenever there is a change to it (since it has to update it when e.g. the language is changed). And also "ImFontAtlas::AddFontFromFileTTF" is not exported, so that shouldn't even build if imgui was overriden with the ReShade add-on variant (should fail with a linker error).
You can however override the font that all of ReShade's overlay is using by updating the config accordingly, though this will require a reload to apply:
reshade::set_config_value(runtime, "STYLE", "Font", (std::filesysttem::current_path() / "GUI" / "resources" / "fontb.ttf").u8string().c_str());