Hi,
You'd have to leave Kansai Int'l for Tokyo, which if your plane arrived at daybreak would take more than 4½ hours, change trains, and take more trains, with good connections getting you into Sapporo at about 10PM at night, for a total of about 14 hours of your vacation wasted on trains and killing the whole day for you. If your plane arrived any later than that, then a same day trip is simply impossible by rail. It'd only make sense if you had a couple of places in between that you'd like to see.
There is a much smarter way, Cindy. You can simply fly from Kansai Int'l to Sapporo on a cheap low cost carrier like Jet Star or Peach, in less than 2 hours and still have the whole day for sightseeing. Plus both go straight from Kansai Int'l. You could still make good savings with a JR Rail Pass for other routes. It seems to me, if you only have 7 days, it is better to use that time for sightseeing than just sitting on trains. It's be kinda like flying all the way to Tokyo but then never leaving your hotel room.
Best of luck and enjoy your stay.