Hello.
You could do that, but it isn't a user-replaceable part. It can be a little tricky to perform and internal components can easily be damaged if you don't have experience.
That being said, any SSD upgrade would be a noticeable speed increase in all tasks. I don't recommend the second SSD in the area under the optical drive, as it will not be secured in place and could interrupt the air flow and internal temperatures of the iMac, causing undue stress on the components and the fans to run at higher RPM to compensate.
If your iMac is a Mid 2011 or later, you would get the same or better speed from an external drive case with an SSD via Thunderbolt connection. You wouldn't have to mod the iMac at all, and could add several (up to 6) daisy-chained Thunderbolt drive as needed.
C