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.
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