Adding minerals back into reverse osmosis water?

I am going to keep the answer brewing focused.

There are a number of salts you can add to get the desired mineral profile you are looking for the most common being added at 1g/USGal :

• Calcium Carbonate    CaCO3 => Ca+2(105ppm) + CO3-2 (158ppm)
• Calcium Sulphate      CaSO4 => Ca+2(61.5ppm) + SO4-2 (147.4 ppm)
• Calcium Chloride       CaCl2  => Ca+2(72 ppm)    + 2Cl- (127ppm)
• Magnesium Sulphate MgSO4 => Mg+2 (26 ppm) + SO4-2 (103 ppm)
• Sodium BiCarbonate NaHCO3 => Na+1 (75 ppm) + HCO3- (191 ppm)

Adapted from www.howtobrew.com.

Balanced Profile

Ion Profile in ppm

Ca2+ Mg2+ Na+ Cl- SO42- HCO3-

80       5        25    75    80     100

As mentioned this is a huge topic. While most brewers (or homebrewers at least) seem to not worry about it much, some do exactly what you're describing to build water with a desired mineral content for each beer they make. Adding sodium chloride, calcium sulfate, magnesium sulfate, and other salts to reverse osmosis water allows brewers to build exactly the water profile they want.

In many cases, though, brewers are looking for ion concentrations that will aid with mashing and fermentation as well as augment the final flavor of the beer, often trying to match historical water profiles. For your purposes you may want something different; it's hard to say. However, with some research you should be able determine what water profile you want and what amounts of which salts will get you there. You can buy the necessary salts at a local homebrew shop and while there you may also be able get tips on how to go about the process.

Yes you can add the minerals in powder form. There is one you already have in your kitchen: NaCl - Sodium Chloride or Table Salt :)

There is a lot of brewing related information on water chemistry. Maybe have a look here: https://sites.google.com/site/brunwater/

The information will be mostly for brewing, but it will also tell you about the salts that you can use. Here is an overview of the common salts: http://howtobrew.com/book/section-3/understanding-the-mash-ph/using-salts-for-brewing-water-adjustment

To find out how much you must use, maybe have a look at a bottle of your favourite mineral water. Normally the tell you what is inside.

If what you are making is homebrew beer and known beer clone, then its not entirely uncommon to copy the natural mineral composition of the water source. In my limited experience this is quite an advanced undertaking. Many seem to use RO water or simply declorinated water for making beer. Me personally, check a recent water treatment plant report and dechlorinate. If needed I might also adjust the alkalinity of the water, but the minerals I keep.