What would the point be? You already have a 32 bit image, and if you want the toning tools, all you need is to convert to 8/16 bit.
And you can reopen a 32 bit image in HDR mode -- because 32 bit is always HDR.
Ah! My apologies. I didn't realize that the toning tools would come up (nor is it mentioned in the Adobe knowledgebase page I read on the subject of converting to 16/8 bit). I guess it should have been obvious, though, that there would need to be some control over how to map the tones. Actually, this appears to be the much better way to work with HDR tonemapping in CS5, since you have a real 100% preview, which is, sadly, lacking in the "real" HDR workspace. Great to have found this out.
Converting to 8/16 bit is the REAL method.
The convenience features were added to MergeToHDR for those who don't know what HDR is but want the look offered by other applications.