Where do you read that you are not allowed to modify to your needs? The whole essence of stock is to buy a licence of a picture and modify the picture to your needs. You have very few restrictions. So yes, you can do that.
Customer support probably misunderstood your need. You need an extended licence for merchandizing. You are not doing merchandizing.
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).
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