Sure ... no problem. However, using an external 2.5" SATA SSD will not have the speed of the internal SSD found in the new iMacs (if ordered as such). It is unlikely you will notice the difference in normal day-to-day usage. You can get a small bus-powered Thunderbolt SSD and simply velcro it to the back of the stand to be out of sight.
Although I ordered my iMacs with internal SSDs, I do this with an external bootable Windows SSD using LaCie Rugged Thunderbolt SSDs (with the orange "lifeboat" removed).