The phrase "add to existing" is correct and usable in written English.
It can be used when referring to the action of increasing or supplementing something that is already present or established.
Example: "We need to add to existing resources to ensure the project is successful."
Alternatives: "supplement current" or "enhance existing".
We have approached and worked with some of the world's largest producers of laundry products but even though the technology exists and could be relatively cheap to add to existing products, it's proved to be a tough sell.
Putting Mr. Ashcroft's proposal into effect would only add to existing fears in immigrant communities.
The findings add to existing research that neonicotinoid pesticides are contributing to the decline in bee populations.
The company will build new plants in Irwindale, Calif., and add to existing ones in Irwindale and Alhambra.
Christmas debt will add to existing average household debt of £6,020 according to the advice service.
This time, the debate centered on what do you spend on new programs or add to existing ones as opposed to what you would cut".
The construction of new churches, so noteworthy a feature of the earlier years, ceased as men did little more than rebuild or add to existing structures.
Discovery says the cams will add to existing deals with some advertisers, and be an enticement to others that haven't bought ads on Animal Planet before.
So there is plenty of time to use existing miles or, conversely, add to existing accounts in anticipation of the combination.
The current cohort of retirees is the first for whom driving was commonplace, so new generations of vehicle-owners will replace rather than add to existing ones.
The company's shares have fallen sharply this week after easyJet warned on Monday that consumer caution following the Brexit vote was likely to add to existing problems such as cancelled flights and concern about terrorist attacks.
