Yes, you can save non-raw as DNG. Why do this?
How do this?
Tested full circle with PSD and jpeg. Motivation for diving in to test was too many archive formats to deal with and a growing appreciation of Camera Raw, plus an article on Linear Raw by Barry Pearson
1. Import PSD/jpeg files into Adobe Lightroom beta.
2. Select Convert to DNG from library menu - in dialog box that pops up uncheck (convert RAW only), check delete originals as you feel safe with
At this point you will have all your PSD/jpeg files converted to DNG format. All EXIF and metadata present. However Lightroom creates a standard size embedded preview which has a maximum height/width of 1024 pixels, ie. it is not a full size preview.
3. Open Adobe DNG converter on the directory of new DNG files
4. Set preferences to create a full-size JPEG
5. Save into a different folder than the original
This is not really for safety but because Adobe DNG converter does not overwrite existing DNG files. Instead it appends a _1, _2 as appropriate. If you have your files already catalogued with iView for instance you'll have to rename them all to match the original catalogued file name. By selecting a different folder you then only need to copy them back to your current working/archive directory. The new preview will then be seen.
All DNG files behave as expected in Lightroom, Photoshop CS2, Bridge and iView.
Usual disclaimers apply. Try this yourself on copies of everything first.