Just don't use water.
I haven't made that kit, but after secondary is completed and you need to top off, use a commercial bottle of any of the varieties, of which that blend is made. First choice is to use that variety, which is the largest percentage of the blend, then work down from there.
After you rack, save your sediment in a separate "sealed" (clear glass if possible) container. Let it settle for a week or so; use a turkey baster to suck the cleared wine off the top of the sediment. You can recover about one extra bottle each time you do this.