"Append" is only available when setting a multi-checkbox – make it available when copying too
When creating a workflow that invloves multi-checkboxes, the native HubSpot append functionality isn't available when you use the "Copy contact property value" button – only when you click the similar "Set contact property value" button. (In other words, if you use the Copy button, your workflow never presents an option to append – it's overwrite or bust.)
Why would you need to copy without overwriting?
Let's say you have a contact property that tracks which products a contact is interested in. With this change, you could easily aggregate info from multiple contact properties into a single company property. So for example, if you know that one employee at a company is interested in products A and B while another employee is interested in products C and D, you can build a workflow to copy all employees' interests to the corresponding company property. The result is that you can see all four products that anyone at the company has shown interest in. Then when your sales team walks in the door to speak with the guy who likes A & B, they won't be caught flat-footed when they suddenly need sales collateral on C & D.
I also think this would be a fairly low-overhead project for the HubSpot dev team. Granted, I'm not a developer, but I'd think the hardest part's already done since the append logic already exists, so I'm thinking/hoping/crossing my fingers that it would be a relatively small-ish project to clone the feature from one button to the other.