Is it possible to enable TRIM for an external SSD?
I realize this question is over 4 years old, but I'm adding a comment because this page was the first hit for a Google search on the subject and some of the comments are outdated whereas others were plain wrong.

Apple added native TRIM support in OS X 10.6.8 but only for Apple OEM SSDs. To get around that restriction, software such as Cindori's TRIM Enabler allowed TRIM to be used on 3rd party SSDs. Later, OS X 10.10 (Yosemite) introduced a security requirement called kext signing which prevented TRIM Enabler from working without completely turning off kext signing. Then in OS X 10.10.4, Apple introduced a “trimforce” command which enabled TRIM on 3rd party SSDs without turning off kext signing. (I have sources but Stack Exchange won't let me post more than 2 links)

Contrary to many of the comments here, the interface DOES matter. The vast majority of USB enclosures do not support TRIM, but allegedly there are some that support it such as the JMS 578 USB bridge controller (source: http://bbs.pceva.com.cn/thread-125643-1-1.html). All five of the USB 3 enclosures / docks I have support it. Firewire supposedly supports TRIM, but I haven't tested it. Most Thunderbolt enclosures should support TRIM, and I currently have TRIM enabled on a SanDisk Extreme SSD in an Akitio Thunderbolt 2 enclosure. I used the trimforce command in terminal to enable it.

In order to check if your enclosure supports TRIM, you’ll want to go to Apple > About This Mac > System Report > Hardware > SATA/SATA Express > and look for “TRIM Support” under the enclosure specs. If TRIM Support is not listed, your enclosure most likely does not support TRIM. If it is listed but says “No”, you can try turning TRIM on by using the trimforce command in terminal. Just open Terminal and type “sudo trimforce enable”. Once your computer restarts, check System Report again to see if TRIM Support changed to Yes.

Finally, TRIM does matter, even for modern SSDs with sophisticated garbage collection. I experienced this firsthand when my SanDisk SSD started slowing down as it got older before enabling TRIM on it. There’s a good, but somewhat outdated article about it here: http://arstechnica.com/gadgets/2015/04/ask-ars-my-ssd-does-garbage-collection-so-i-dont-need-trim-right/