I think in pounds, so allow me to convert. You are considering adding roughly 1.3 pounds of 50/50 wheat LME to your barleywine wort. I do not think you have much to worry about for clarity, especially with the length of time a barleywine conditions. I also do not think you have to worry about that little amount of wheat affecting your flavor and aroma profiles.
That said, though, I hate jury-rigging recipes at the last moment just to reach some target volume and gravity. I would rather forgo the other two liters and work with the wort you got from the mash, adjusting the sugar and date syrup amounts accordingly.
Beer is good for anything from hot dogs to heartache.
Drinking Frog Brewery, est. 1993