The only way to officially allow Apple to install an SSD for you would be to have an iMac that came with SSD in the first place.
Upgrading the HDD to the SSD technically voids your warranty, but if you manage to swap the HDD for SSD without damaging anything, they have no ground in which to consider your warranty voided. You'd just have to replace SSD with original HDD if you ever had to take it in for something else.