Okay, so it's actually easy but not easy.
You can wire the negative voltage to the center pin and the positive to the sleeve on your DC jack. That'll work perfectly fine with any standard Boss style adapter. But you can't use a daisy chain adapter or something like a Boss DC Brick with this effect and any standard negative ground effects. Some will detect the short and shut down to avoid problems. Cheaper adapters, who knows. Maybe you'll start a fire.
There are workarounds, though! You can use a charge pump. There are two options here. If you use a MAX1044 or an ICL7660 or an LMC7660, it's reeeeally easy and requires very few components and there are a bunch of schematics available. These are all around $1-$2 per unit on eBay if you order from China. You can get free samples of the LMC7660 from TI.com. These are good options. The MAX1044 is allegedly pretty sensitive to static shock, heat, and will fry if you feed it over 10V. The 7660 is a bit cheaper, sturdier and I THINK can handle 18V in but I don't know for sure off hand.
The NE555 can also be used to do this, though! You can get like 50 NE555s for the price of 2 MAX1044s on eBay. It's very cheap. And it's a timer chip rather than a charge pump so there are a bunch of other things you can do with it. Not so many related to audio, but you can make LEDs blink and stuff. Here's a layout that I know for a fact will work: http://tagboardeffects.blogspot.ca/2013 ... erter.html
Here's the Dutch site that has the schematic that inspired that:http://www.circuitsonline.net/schakelin ... ne555.html
If you use Google Chrome, it'll prompt you to auto-translate for you. Pretty slick. Anyhow. All of the above can ALSO be used to turn 9V into 18V or higher. The current draw from using a charge pump increases. Not to the extent that you'll run into problems with DC adapters, but you'll drain 9V batteries noticeably quicker.