Hi and welcome, Edward.
If you haven't already, read the stickies at the top of the
Making beer forum, particularly
Simple things that make HB better.
It is possible, but difficult, to make a true lager with a kit. You need to be scrupulous with cleanliness, use a true lager yeast and ferment at very low temperatures (9C-12C) to get that clean, crisp lager taste. Brewing at a higher temperature will result in a beer that's more fruity and like an ale.
Basically all adding dextrose will do is increase the alcohol content. If you want to add body to the beer then add, say, 500g dextrose and 500g of light dried malt. (Or use the recommended Coopers Brew Enhancer with the can of pilsner.)
Depending how it turns out you can adjust the amounts next time.
Hope this helps.
Cheers,
Oliver