ro/di

how does one go about adding stages ie a 3 stage and turn it into a 6 or better ?

 

I don't know. Someone will.

 

Buy a canister (the assembly the filters fit in; may be called something different), and plumb the output of your system to the input of the canister.

 

Ditto

 

It is far more efficient to improve the filter cartridge in you existing canister than to just add stages. Stages don't mean a hill of beans, its what the stages contain that is important, not how many. The word "stages", is an ebay sales ploy to make cheap drinking water systems sound really cool but in reality 6 or 7 mediocre stages is not nearly as efficient as 3 or 4 high quality stages.
For RO only you want 3 stages, for a high quality RO/DI only 4.
You need one good prefilter in the 1 micron or lower range, 0.5 microns is even better. One 0.5 micron Chlorine Guzzler style 20,000 gallon carbon block the the RO membane itself. If its an RO/DI you add one additional 10" vertical canister with a refillable DI cartridge filled with good mixed bed DI resin and you are set.
Additional stages can actually be a hindrance since each stage adds additional pressure drop to the membrane. If you really want to improve the water quality add an additional DI, not prefilters or carbons which do little for water quality. Dual carbons go back to the day when carbon was not as developed as it is today, some older carbons were exhausted in as little as 300 total gallons so they had to double up. Today some vendors still use dual carbons but they are using a high micron prefilter which forces the carbon block to act as a secondary prefilter which plugs or fouls the pores and renders it useless for chlorine and VOC adsorption. Stick with a good prefilter and single good carbon and you don't have that issue.

Check out the prices on the 0.5 micron absolute rated prefilter, o.5 micron carbon block and the SilicaBuster DI cartridge here:
SpectraPure Customer Appreciation SALE! 20% - 50% off

For about $43 you can have all three and an additional caniste ris not that expensive if you need one of those.