Post mix to make soda?

Okay-- not beer but....

SWMBO was asking me the other day if it would be possible to have the occasional soda on tap on my kegerator (for family-type functions). So I was at Sam's and noticed that they sell the box syrups for a post-mix soda system. Which led me to...

It seems to me that what is missing from a post mix syrup bag is water and Co2. So in theory I should be able to take a post mix bag (runs about $7 and is 1 gallon to make 6 gallons) pour (most of) it in a corney keg (yeah, yeah-- probably need to replace the rings after), add some water and then force carbonate.

Aside from the obvious clean up issues--- is there something I'm missing that would make this not work they way I think it would?

I have a keg of soda water in my kegger (about 30 psi). I keep syrups in the fridge, so all I need to do is drawn a pint of bubbles and add syrup & ice.

If you want to pre-mix, just be certain to sanitize well. If yeast gets into the soda, it will eat the sugar and blow the safety valve. Leaving you with highly carbonated, unsweet not-beer, not-soda.

what syrups do you use? Do you buy bag soda and portion it into bottles?