You could use it to dilute your tap (or bottled) water but if you used 100% RO/DI then you'd have to 'rebuild' it with salt additions. I used bottled water and dilute bottled Spring/Drinking water with Purified/Distilled water when I make light-colored beers (the dilution is to reduce the alkalinity). Then I usually add some salts to get the hardness back up.
It's great that you already have a source of RO, but I'd recommend getting your tap water tested for it's current makeup so you'd know what kind of mix to use.