Anything you add will change the gravity and change the flavor. High fermentable sugars will make the beer taste a little dryer, maybe you can try a combination of dextrose and DME or something like that.
Any change in the recipe is going to change the flavor profile to some degree. Using a maltose-based source (DME or LME) is going to up the body slightly since yeast won't ferment 100% of it. Using simpler sugar (corn sugar, turbinado, candy sugar, honey, anything like that) is going to ferment out entirely, and can actually give the beer a lighter, drier flavor. It'll also have an effect on perception of the hop character. It ultimately depends on what you're going for.
Ultimately, the best route if you want a higher ABV brew is to go with a recipe designed with a higher ABV in mind.
Well I'm focusing on high gravity mainly because they are hard tsupposo get a hold on here in tn. But I love 1554 but want my clone to have a punch. Have any iof you ever tried brewbrothers.biz recipes? They have a double IPA that's supposed to finish out at a whopping 9.3%! And an imperial porter that finishes at around 8 something. Wonder if they are any good