I have the second half of my hard drive encrypted. Now I want to encrypt the first half as well. Being the second partition I can't extend the LUKS partition. Can I add the first partition to the existing LUKS or do I have to set another LUKS for this one partition?
You'll need to create another LUKS container.
Your LVM volume group can span both LUKS containers. You'd simply create an LVM physical volume in the LUKS container and add it to the LVM volume group. However, that means both LUKS containers would need to be unlocked before you can bring up the LVM volume group.