Hello HubSpot Community,
Thank you as always for your continued feedback, it helps us build a great product.
We are constantly evaluating and re-evaluating our priorities and roadmap to deliver as much value to our customers as possible. It is currently possible to append data to a multiple checkbox property. Here is an article that describes that functionality. However, at this time, adding additional functionality to our import tool is not something our team is currently planning to build natively into HubSpot. This is certainly subject to change in the future and we will update this idea if that becomes the case.