ComboBox and original value as an array

I have a legacy MongoDB database (collection) where there is single value stored as an array List . But its only a one value that must be, in UI selected with ComboBox. So I have a model beanclass

Project { List<Company> companies; }

And I would like to bind it and edit with VAADIN ComboBox. Initialy I thought I can use some customer converter for ComboBox but can't get it to work. Combo should edit the first value in the List (Company bean) and store back into companies field as an array to remain compatible. Is it event possible to do it and if so can you give me some hint how to accomplish this?