Add the task to your package. Then "set" a new value and you can basically pick any property of the package, including properties of any task already there. Select the property and then you can choose how the value is set in the right pane, from INI, query, etc.
Not sure if you can change a global and then use that to change the ftp task in the same dynamic properties task. I assume it processes top to bottom, but you need to test it.
If not, string 2 dynamic properties tasks together using the workflow, then link the 2nd task to your ftp task.
At home today and have to go, but I'll try to mock something later and post it for you. Ask more questions if you need to, not sure if I'll be here later, but Andy might.