Adding a field to a table (but in another database)
You might be able to get away with just adding a new schema to the existing database, but either way the solution is approximately the same.  

In order to avoid interfering with the dacpac deployment, you may want to avoid actually implementing a true foreign key constraint.  You can either make a stored procedure do the inserts/updates and have it manually do the constraint checking before inserting a new row.  

But basically, create a table with a non-incrementing primary key (that should have a 1:1 with the source table.  Then add your custom fields to that table.  You can use a view to merge them together to make it easier to digest the contents if you want.

CREATE TABLE dbo.Extension
    (
    DatasetID INT NOT NULL PRIMARY KEY --make sure to match type to source
    , InventoryCount INT NULL 
    )


As I said, if you can add it as a custom schema (assuming that survives the dacpac update process the vendor is using) then it can live inside the same database (my preference).  But you can add it into a new database if you want as well.