I've tried this twice, both with dry yeast straight from the packet. One was a pseudo-lager that I cold conditioned for a week before bottling so the additional yeast was just for insurance. This beer turned out fine. The second was for a strong IPA that I thought was done fermenting. I had about 2 pounds of unrefined sugar in the recipe (similar to belgian recipes). After a week of bottling I thought the beer was perfect. I let it hang out for another week or two before I put it in colder environs. Bottles ended up super carbed and undrinkable. The beer is much better when decarbed so I don't think it was infection.
The "bottling yeast" was different from the fermenting strain and with the amount of sugar I thought I may have stalled the fermentation and the new strain ate the residual sugars on top of my primings (which were to 2 volumes CO2). Others have said the added yeast provided nucleation points for CO2 and that's why I have (slow) gushers. I'm not sold on either theory yet.
While you probably have enough yeast already, the method you are choosing is what I'd do next time. Good luck and report back!
Fermenters: Helles, Oktoberfest
Bottled: Old Peculier, Dopplebock, Belgian ale with homemade candi syrup (2 varieties), Berliner weisse
On Deck: Hefeweizen