Creating a button that adds input text to an existing variable

I'm trying to create a set of buttons that, if selected, will add a piece of input text to an existing variable.  

Here's a bit of background ...

On the first screen, the user can enter up to eight tasks in the text entry boxes on the left. The user then checks one of the four buttons next to it. They can choose between urgent, important, both or neither. They can only tick one. (See first img1)

On the second screen, the tasks should be organised into the four boxes, depending on which button was checked on the previous screen. 

So far, I've figured out how to change the properties of the variables I've labelled as %Box1%, %Box2%, %Box3% and %Box4% to be equal to the contents of the first text entry box. (See second img2)

i.e. Whatever is entered into the first text entry box will appear in the box that is linked to one of the check boxes.

What I need the interaction to do is add additional string of text to the appropriate box, depending on what is checked. 

I've had a look at Adjust Variable in the Triggers panel and know that it's got something to do with this area, but I'm unsure how to proceed.

If anyone give give me some insight, suggestions or alternative ideas  for creating this interaction, I'd be very grateful.