Looks like a proprietary MS format (Microsoft Localization Studio)
Over the years I've tried to get a copy to play with it myself, but I didn't manage.
(not even when I was an MS i18n MVP :-)
Apparently MS provides it to translators (translation companies?) while signing an NDA, and they are not allowed to share / talk about it.
Some sites seem to indicate that the file and also contains dialog resizing information for .rc files.
(or one of the docs from Mr. Wasala? I don't know, I've spent too much trying to read all I can find about it, which is not much)
So, short story: proprietary, undocumented, secretive format, restricted access to the tool producing it.
I think the odds that Okapi or someone else can convert that to XLIFF / back are slim to none.
I suppose there is no official way to get such files other than from MS.
And in that case MS would also give access to the tool.
Mihai