I'm still learning to do post work myself as I don't really use it on any of my renders, so I'm not too sure the best or easiest way to go about it. From what I have briefly seen on youtube you could just put the render without the hdri (so the sky comes out as grey) on one layer then put whatever you want to use as the sky etc on the layer underneath? you probably can't use they HDRI you have in DAZ as I think is 360 and would just come out stretched, but I'm sure you could find some free sky images on the net somewhere.
Sorry I can't be too much help, if anyone has better knowledge with GIMP or postwork can you please advise