Can I decrypt and read a partition without booting it from another mac? I have a HDD with a OS X data partition on it that is encrypted with FileVault. It also contains NTFS and ext4 partitions. This is not a boot partition. My boot partition was on a separate SSD. I can not use the boot partition to boot OS X anymore.

Can I take out the HDD with the encrypted partition, plug it with a USB dock into a macbook and unlock/read the data?

NOTE: I just need to recover one small folder from the partition that I could transfer on a usb stick.
You can unlock/decrypt and read any FileVault2 volume – either system or data partition – as long as the invisible Recovery HD partition (for a bootable system partition) or a Boot OS X partition (data partition) is still present and a password is known. Both partitions are usually following the respective encrypted partition. Boot OS X is no bootable partition as the name may suggest but a 134.2 MB partition probably holding some informations about the encrypted data partition.