As Weaselboy said above, TRIM doesn't work over USB3.
BUT... chances are the "lack of TRIM" probably won't affect your performance in any noticeable way. Some newer drives use it, but others don't seem to benefit from it at all.
I've been booting and running a late 2012 Mini since January 2013 by using an SSD mounted in an external USB3/SATA docking station. I've been using a Crucial drive in the dock for the better part of a year now, and it runs as fast now as when first installed. As far as I'm concerned -- and it's my opinion only -- the lack of TRIM is a "non-issue", and I'm not going to worry about it.
Installing an SSD into a USB3 enclosure (or dock) that is specifically stated to support UASP (USB attached SCSI protocol) is a quick, easy and relatively cheap way to obtain a BIG BOOST in speed without going through the trouble of prying open the iMac.
You can put the OS, apps, your accounts on the SSD.
You would probably want to keep any large libraries (such as pictures, music and movies) on the internal HDD (to leave space available on the SSD for swap files, caches, temp files, etc.).
I predict you will like the speed increase very much!