1) Backup your neighborhood before trying this technique.
2) Ensure that your lot has enough room for the new road: one additional square in the neighborhood view, or ten additional tiles in the lot view.
3) Open the neighborhood in SimPE.
4) Determine the orientation of the lot (U11).
5) DO NOT, under any circumstances, change U11. This is very important. If U11 doesn't actually match the data in the lot file, you can cause major problems.
6) Determine where you want the new road, based on the orientation of the lot and the current road location (U10).
Here's a trick that I use to get the values right:
First, draw a very rough sketch of your house and the current road, viewed from above, on a piece of paper.
Then, rotate the piece of paper so that the road is at the left, right, top or bottom of the page, depending upon the old U10 and U11 values.
Now, keeping the paper in the same position, mark where you want the new road(s) to go. Each new road will either be at the left, right, top or bottom of the page. From this, we can determine the new U10 value, by adding the U10 values for each road location.
7) Change the value of U10 as discussed above. SimPE works best if you leave the existing "0x0" prefix and just change the last hex number.
8) Commit the change, save the neighborhood file and exit SimPE.
9) Start the game and go into the neighborhood.
10) Use the "Move or Rotate Lot" tool in the "Lots and Houses - F2" menu to move the lot so that it "snaps" to the appropriate roads.
Voila! You now have a corner lot.
Please note, I just tried this technique on Don Lothario's house at 150 Main Street. I used the LotExpander to increase the lot size by one on the right side (as viewed from the road), then changed U10 from 0x04 to 0x06. It worked like a dream.