Sure. Just assign it.
psf = [1,2,3;
4,5,6;
7,8,9]; % or whatever it is.
Do you know the size of it (the window width)? Do you have a formula for it? You need to or else how will you put the values into it. Then you can use conv2() to convolve your input image with your custom PSF.
blurredImage = conv2(inputImage, psf, 'same');