Increase the number of rows for a Mat without touching original data

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So let's say we have the following code:

// Let's assume imageData represents an image I get as a parameter
unsigned char* imageData;

// Let's also assume that the image represented by imageData is of the exact same size as the parameters we pass to create the mat called x. This means the mat is perfectly filled up.
Mat x(height, width, imageData);

What I want to achieve is to "increase" the numbers of rows in the mat "x", without touching the original data. I tried the resize function but I noticed it stretches the data. So the result is still a mat that is completely filled up. What I want after increasing the number of rows is that the original "height" number of rows still has the imageData untouched, and the newly added rows is empty. I'm experimenting with this due to efficiency. What I have right now is I simply use push_back to append new data to the original Mat to resize it. So I have my original Mat x, then I append a 2nd Mat y (which has the same number of columns as x) to it.

The data I insert into my 2nd Mat y is individually done per cell using the at function, so I was thinking if it was more efficient if I can somehow just increase the number of rows in the original Mat x and use the "at" function on x directly instead of using a temporary Mat y in order to use push_back.

I have also tried this:

// imageData represents an image that has a lower height than the Mat it's being passed to. I assume the bottom set of rows would just be empty, but when I tried to access it near the bottom part I'm getting segmentation fault errors for some reason
Mat x(height * 1.5, width, imageData);