"A lost cause, that is."

Sure, they're correct. The appended "I say" tends to sound a bit haughty or archaic, while the "that is" sounds a bit colloquial, but both are well within the bounds of accepted usage.


That is probably not a good habit. It certainly will make you sound more "foreign."

Such phrases aren't confusing, but they don't seem to express anything. Few people nowadays add such flourishes; common ones I can think of are man, dude, yo, or no? — all of which relate to some kind of stereotypical, hackneyed speech.

By the way, the idiom is lost cause, not lost case.