You can but not out of the box. In order to put a SSD into an iMac that isn't a 27" or a 2011 21.5" then you need a optibay. This replaces your super drive with a SSD so you loose the use of the drive but you get a SSD. This allows you to have your standard HD with an SSD. If you want to find on here is a link to one that OWC sells that is relatively easy to install.
Hope this Helps.