Adobe DNG Converter can do this.
It's freeware and publicly available from Adobe website.
Nice GUI and many options ...
But latest dng_validate.exe still can be used to compress DNG files, see -dng option.
I mentioned that I couldn't find an option to compress in Adobe DNG converter. It looks like that option has been removed in the latest version unless I just can't find it. Does anyone have an older version installation file for DNG Converter that still has the compression option?
As for dng_validate.exe, if I have an existing image.dng, then do I just put:
dng_validate.exe -dng image.dng
and then I'll end up with a compressed image.dng.dng? I purposely put the extension twice because dng_validate.exe says this:
-dng <file> Write DNG image to "<file>.dng"