Hope your eyes don't bleed.
This is a very draft mockup, the main idea is to get 2 boxes for sample loading:
1- upload CSV
2- browse unused sample
For 2 we can have a modal (maybe similar to the date picker in the scheduling section) in which the user can browse the finished surveys exposing some basic information like the amount of unused respondents. The the user can select the unused sample that will be loaded into the new survey