AddColumns doesn't change the collection that is passed to it - instead it returns a new collection with the newly added column(s). What you likely need to do is to have the extra column when you are creating your local collection, and then use the UpdateIf function to set the values in that column.
In the attached app, I'm using the following formula to initialize the local collection:
ClearCollect(
ScoobyDoo,
AddColumns(
Table(
{Id:1, Name:"Scooby Doo", Age:7, Human:false},{Id:2, Name:"Shaggy Rogers", Age:17, Human:true},
{Id:3, Name:"Fred Jones", Age:17, Human:true},{Id:4, Name:"Daphne Blake", Age:16, Human:true},
{Id:5, Name:"Velma Dinkley", Age:15, Human:true}, {Id:6, Name: "Scrappy Doo", Age:3, Human:false }),
"Species", ""));
UpdateIf(ScoobyDoo, true, { Species: Blank() })
Notice that I first initialize the "Species" field with an empty string (to define that the column is of type text), and then I clear the value (since I don't have anything there).
In another button, I can then update the collection based on a specified condition:
UpdateIf(ScoobyDoo, Human = true, { Species: "Homo sapiens" })
