You can, but it's not that simple. Follow these steps below,
1. Give the STEAM voucher code to your friend
2. Either make a dummy PSN account or have his PSN account activated on your PS3.
3. Play the Portal 2 PS3 disc on your PS3 using his dummy or his PSN account
4. Press SELECT to open STEAM while playing Portal 2.
5. Link his PSN or dummy account to the Steam account he has.
6. There ya go, you can now play together!