There are a few ways to sweeten wine: Filtering, Mutage, too much sugar and adding something sweet after the wine has finished. Commercial wineries do not use sorbates to kill the yeast and preserve some of the sweetness. It screws up the flavor.
By far the most used way to preserve sweetness is to filter a wine either before it's done fermenting or add sugar and then sterile filter. Sterile filtering in this case is .3u (micron). This is also done in conjunction with a pretty high dose of potassium bisulfite to keep the little bugs from refermenting again.
Mutage is a french term for adding alcohol to a still fermenting wine (think Port or Madeira) and can make wonderfully sweetish wines that last for hundreds of years. I tasted a Madeira from 1780 a couple of years ago. It blew my mind. No filtering necessary here since above 20% alcohol it kills the yeast.
You can keep adding sugar until you reach the theoretical limit of the yeast, but you will have problems with this as the yeast will keep trying to ferment slowly over many years and you are likely to end up with bottle grenades randomly exploding over time. Since you can't get it over 20% naturally, there is a space between around 17% and 20% where the yeast is kind of still alive trying to ferment.
You could also suppress the yeast with a really high dose of potassium bisulfite, but then that also gives off flavors.
I think you have to choices. Either bring the alcohol level above 20% with vodka (you will have to figure out the math on that) or just add a bit of sugar to your bottle before you want to consume it. Without investing in a sterile filter, this is all you really have.