Решил данный вопрос таким способом:
// load image from file
System::Drawing::Bitmap^ imageCLI_RGB = gcnew System::Drawing::Bitmap^(L"Lena.jpg");
// resize image for change RGB to RGBA values
System::Drawing::Bitmap^ imageCLI_RGBA = gcnew System::Drawing::Bitmap(imageCLI_RGB, 640, 480);
// lock the bitmap's bits.
System::Drawing::Rectangle rect = System::Drawing::Rectangle(0, 0, imageCLI_RGBA->Width, imageCLI_RGBA->Height);
System::Drawing::Imaging::BitmapData^ bmpData = imageCLI_RGBA->LockBits(rect, System::Drawing::Imaging::ImageLockMode::ReadWrite, imageCLI_RGBA->PixelFormat);
// get the address of the first line.
System::IntPtr ptr = bmpData->Scan0;
// declare an array to hold the bytes of the bitmap.
// this code is specific to a bitmap with 24 bits per pixels.
int bytes = System::Math::Abs(bmpData->Stride) * imageCLI_RGBA->Height;
array<System::Byte>^ rgbValues = gcnew array<System::Byte>(bytes);
// copy the RGBA values into the array.
System::Runtime::InteropServices::Marshal::Copy(ptr, rgbValues, 0, bytes);
/***** ALL MAGICK HERE *****/
// RGBA contains CLI char array to simple char array
pin_ptr<unsigned char> p = &rgbValues[0];
unsigned char* rgbaTable = p;
// add RGBA values to ImageMagick image
Magick::Image* imageIM = new Magick::Image(imageCLI_RGBA->Width, imageCLI_RGBA->Height, "RGBA", Magick::CharPixel, rgbaTable);
// blur image
imageIM->gaussianBlur(3, 2);
// copy values back to the RGBA array
imageIM->write(0, 0, imageCLI_RGBA->Width, imageCLI_RGBA->Height, "RGBA", Magick::CharPixel, rgbaTable);
delete imageIM;
/***** END MAGICK *****/
// copy array to image
System::Runtime::InteropServices::Marshal::Copy(rgbValues, 0, ptr, bytes);
// unlock the bits.
imageCLI_RGBA->UnlockBits(bmpData);