Yes, you can. But you can't just back-up only those games to the flash drive, you must back-up all data from PS3 Hard Disk Drive (HDD) to the flash drive. Those games will also be backed up along with all other data on your PS3 HDD. See "Source 1" for details.
Instead of backing-up your games, delete them from your PS3 HDD, you can download the same games for free again as long as you are a PlayStation Plus member. Just make sure to download them and a red PlayStation Store icon appears on them on the PlayStation Store. You can cancel the download after making sure of these two things stated in the previous sentence. See "Source 2" and "Source 3" for details.
Q: How long do I get to keep the games in the Instant Game Collection?
A: Games in the Instant Game Collection are free to download with a Plus membership. As long as you're a PlayStation Plus member, you'll always have access to any game you've downloaded in the collection, even if you've deleted a game from your hard drive previously.
Q: When does new content rotate into the Instant Game Collection?
A: At this time, we add new games to the collection every month. This could change and we're always looking at the best way for gamers to experience PlayStation Plus.
Q: If a game rotates out of the collection, can I still download it free with my Plus membership?
A: If you did not download the game when it was in the collection, you will not be able to secure your free download. However, if you downloaded when it was in the collection, but it's no longer on your hard drive, you can visit the PlayStation Store and re-download it anytime, as long as you're a member.
Q: How many times can I download / re-download a game in the Instant Game Collection?
A: You can download a game as many times as you like, even if it's been deleted from your PS3 HDD, as long as you're a Plus member.
Q: What happens to the games I downloaded from the Instant Game Collection if I decide to not renew my membership?
A: You will not be able to access free games downloaded with Plus if you choose not to renew your membership. However, if you decide to re-activate your membership, you will once again be able to play all games downloaded from the Instant Game Collection.
This fully answers your question.
See "Source 4" for latest updates on PlayStation Plus on the US PSN Store.