Is it possible to add an SSD module to an iMac 27" Late 2012 model to create a fusion drive on that?
The Fusion drive was an option for the 2012 27" iMac, so there may be space. The iFixit tear-down for your model shows a caddy that holds both a hard drive and SSD: you'll need to read through it to see whether you're comfortable with taking it all apart -- and more importantly, getting it back together.

If you're not comfortable with the task, then you may want to run an external SSD via USB3 or a Thunderbolt dock of some kind. That should still be faster than the internal hard drive.

I concur with the comment that really, SSDs are cheap enough to buy sufficiently large volumes, making Fusion drives an unnecessary complication.