Is it true to get a higher S.G. you can by add extra Suger and fermentables like extra suger DME. I 'm trying to find a way to raise, The S.G and F.G to get a higher ABV.
One can add any (sensible) soluble substance to get a higher S.G. If the substance is not fermentable then one also generally gets a higher F.G. If one wants a higher ABV then one adds a fermentable substance to the initial brew. This can be done by adding sugar, glucose, golden syrup, molasses (eg.for stouts) candisugar (eg Belgian ales) and malt extract (dry or liquid).
If one wants a higher F.G and ABV then a good option is to use malt extract as only a proportion is fermentable. What ferments boosts the ABV what remains boosts the FG. If one wants a higher S.G and ABV but a commensurately lower FG then use sugars (glucose/dextrose being a nice option).