An enlarger is a great point source for contact printing. The only thing diffusing it will accomplish is to lower the output, and eliminate any spots from giant dust on condensors, assuming you could focus them. I'd rather clean the condensor. If your enlarger is so out that the light needs to be diffused to be even, well, that could be... I've had one like that
If you need to slow it down, stop down the lens and/or raise the head.
unless i am printing with slow paper ( azo )
i use an enlarger, stop the lens and raise the head --- works perfectly.