Can I add an extra ssd to my 2015 iMac 27 5k?

Yes, but no.

This model has one SATA connection, which is where the HDD is connected, and the option of adding an Apple SSD PCI-e blade drive. Not at all the same as a sata SSD. See this guide where it shows the replacement of a blade drive: iMac Intel 27" Retina 5K Display Blade SSD Replacement

It seems to be a high difficulty repair because of the location of the PCI-e slot. It will also be a more expensive SSD to invest in, but there is no denying the advantage of having your OS booting from an internal SSD.

You can always use external ports for an external SSD and launch all your software from there as an alternative for a cost effective, and virtually work free solution.