1. Empty your pouch into a bin.
2. Place your pouch into a bin.
3. Now your pouch slot is empty.
4. Pick up a 50 slot bin.
5. Now you have a 50 slot backpack.
enjoy!
Edit: You can still put 4 more pouches in your other pouch slots as well.
Sometimes its fickle though. It will let you pick up scrap from junk heaps and other times it won't. They should call the resource gathering Scraping and keep the skill Scavenging the same.