When I calculate a solution of 10l at a gravity of 1.100 (which is already big), then I get an amount of 8.4 l water with 2.6 kg sugar.
But that is already stretching things, because without head space your fermentation vessel will probably overflow from kraüsen.
Even for a 25 liter vessel it seems limited (but I do not know the amount of kraüsen generated by such yeasts). At 18 l water and 7 kg of sugar, you get a total volume of 22.3 l wort with a gravity of 1.119. This only leaves a head space of 2.5 l.
You could also stagger your additions, but the alcohol in the smaller vessel will probably exhaust and overpower the yeast at a certain moment.
Further calculation based upon comments
7 kg of sugar has a volume of 4.46 l, so you could still add 5.54 l of water. I see that the yeast has an alcohol tolerance of 18%. With these parameter you would start with a gravity of 1.260. However, from the calculations found at enter link description here it would seem that this would be enough to get to 35% ABV (calculated two different ways). Since your yeast is tolerant to 18%, you would end up with a mix of alcohol and sugar which you could not further ferment.
However, half the amount of sugar in the same amount of water would for certain start to ferment, and with a little luck ferment out completely. Keep your yeasts at a nice temperature of around 22° C, and supply them with a bit of supplements.