I'm not an OpenCV expert. However, as per this page Data Structures — OpenCV 3.0.0-dev documentation, I guess you can use following APIs to assign cv::Mat to ocl::oclMat.
//! assignment operator. Perform blocking upload to device.
oclMat &operator = (const Mat &m);
//! performs blocking upload data to oclMat.
void upload(const cv::Mat &m);
Thank you Dipak