How to mass update 'interests' in a multi-select field

I have just migrated to Zoho CRM and am very happy on the whole, but am trying to work out the best way forward with a thorny issue. I'm sorry if this is a really obvious question to you more experienced Zoho people...

For all of my contacts, I record what their interests are. This is used to decide what articles or news items to send to them in Zoho Campaigns. I set this up as a multi-select field 'News Interests'.

I've found that this approach is pretty neat and works really for building custom views and mailing lists.

However I realised there is one really big flaw: I cannot add a new interest to a selection of people as a mass update. I cannot see a way to do a mass update where I add an item to the multi-select list without removing all the other existing entries.

So if Person A is interested in X and Y and person B is interested in Y, then I want to do a mass update which results in Person A being interested in X, Y and Z and person B being interested in Y and Z.

Am I trying to do something that is impossible or am I missing a trick? Perhaps someone can recommend a better way of recording people's interests which is flexible and works well with mass updates?

Thanks