I am looking to add a value to a multi-select lookup field.

I have a employee record with a list of countries that the employee work in. We have a new country and I am looking to automatically add the new country to every employees record.  I am looking to do this via a workflow script excution.

It would appear that I need to use the following. 

"<picklist field>:ui.add(value); " 

e.g. employee.country:ui.add("UK");

However the interface reports the following error. 

'ADDTOPICKLIST' can be used only in on load actions

How do I add items to a picklist using the scripting language.

Any advice welcome as I cannot find in my documentation search

Tony