You can use an external SSD drive to boot OS X from. Depending on the model year of the iMac the speed difference between internal and external SSD can be pretty dramatic. The late 2015 iMac SSD speed is about 1500MB/s for the internal vs. ~500 MB/s for an external SSD in a USB 3 enclosure. Something to consider when using SSD in a USB3 enclosure, TRIM will not work.