Change to dynamic child variants. Just exchange in your product the customfield, takes per product 10 seconds.
and actually,.. I think you should directly start with vm2.9.8a. The core is at least 99% stable as the vm2.6.x latest. There is no add to cart button in the category view of vm2, because it can make trouble with customfields, in your case it should work, cause you work with childs. But vm3 is just so much better, imho :-) and a lot faster