The inverse of SDL_GetModState, SDL_SetModState allows you to impose modifier key states on your application.
Simply pass your desired modifier states into modstate. This value my be a logical OR'd combination of the following:
typedef enum {
KMOD_NONE = 0x0000,
KMOD_LSHIFT= 0x0001,
KMOD_RSHIFT= 0x0002,
KMOD_LCTRL = 0x0040,
KMOD_RCTRL = 0x0080,
KMOD_LALT = 0x0100,
KMOD_RALT = 0x0200,
KMOD_LMETA = 0x0400,
KMOD_RMETA = 0x0800,
KMOD_NUM = 0x1000,
KMOD_CAPS = 0x2000,
KMOD_MODE = 0x4000,
} SDLMod;