You could use a list view so each line of the grid would be an entity
Then set the last value in the gird to have a flag and user visibility on buttons to only show the add button on the last entry
I would probably go with source microflow on the list view which would sort the values as you want them and set a flag on the last value. Then make sure the add and remove buttons refresh the client as needed.
You would possibly need some css or layout grids to things to stay in line