18%? that may make your wine very un-winelike. rocket fuel, it may end up very harsh and not palatable. alcohol has a tendency to thin out the flavor profile of wine too.
but generally, you figure out what starting SG reading you need to get to the final alcohol level you desire and then add enough sugar in the form of simple syrup or bulk during the kit reconstitution process to get the desired starting SG.
grapestompers has a nice alcohol percent calculator
you can plug in your test numbers and temps to determine your target starting SG.
some yeasts are not high alcohol tolerant, but many kits ship with EC-1118 which can do well in high alcohol environments up to 18% or so.
both of the kits you mention are smaller volume kits in terms of concentrate and dissolved solids so they may not be ideal candidates for high-alcohol wines. some might complain the sangiovese to be thin when made to kit directions so increasing alcohol another several degrees cold really magnify that effect.
keep in mind too that while most kit instructions indicate final SGs of 0998, quite often your wine ferments beyond that, sometimes as low as 0992, which affects your final ABV calculations. this makes accurate predictions of final alcohol a bit of a guessing game until the wine is finished fermenting. most commercial wines with a few exceptions run in the 11.5%-14% range.
more pleasure to the palate can be had with the addition of zante currants or banana soup, biolees, and other kit tweaks over adding sugar to get higher abv readings. IMHO anyway.