It's a far more complex solution than you might think. IMHO, THE King is not inclined to just add sweetener from what I can tell. In fact he adjust the components of the mix so it taste sweeter and this understandably places a limit on the sweet factor he can produce (Sharon PR Lady extraordinaire mentioned this as well with an email exchange with THE King). Obviously this is just my guess and I'm by no means an expert DIY geek.
For example, I'm currently vaping Lime Cream. YUM! You could increase the % of Cream and Lower the % of the Lime extract to achieve this. Just because it's described as "cream" doesn't necessarily mean it doesn't have other elements in it and it's not sweet all by itself. We have had a few posts on using liquid Stevia like this one:
http://www.e-cigarette-forum.com/fo.../446286-ahlusion-e-juice-79.html#post11017076 The "Clan" members get good results from what I read so that would be my first stop.
You can achieve less than one drop in your mix using a toothpick swirled in the Stevia liquid and then into your juice. Start with a small amount and work your way up. Ideally, this mix should be mixed in a separate container which you shake vigoursly before you fill your device.
In the DIY world these are often used DURING the creation of the mix and normally not as an afterthought:
Cotton Candy (Circus) (TFA)
TFA - Sweetener
HTH and happy mixing