Bulk update multilist

It's great you've established custom properties of multilist type! However, what I don't like, if I want to bulk update list of records, it updates all settings with no option to append value to existing settings.

Example:
Incidents have custom property Tags with values: property1, property2, property3, property4, newProperty.
There are incidents:
- Inc1 - property1 associated
- Inc2 - property2 associated
- Inc3 - property3 associated
I want all of them to be associated additionally to newProperty tag. Select them all on the list, for Inc1 append newProperty, click on 'Fill with this value' and... now I got:
- Inc1 - property1 and newProperty associated
- Inc2 - property1 and newProperty associated
- Inc3 - property1 and newProperty associated
While I wanted:
- Inc1 - property1 associated
- Inc2 - property2 and newProperty associated
- Inc3 - property3 and newProperty associated

Can you think about some solution/workaround? I don't want to have large number of customproperties, thus 'Tags' sounds like a good solution for temporary labelling, however manually appending new tag could be painful if applied to hundreds of records...