The new iMac 5k has two drive connections a SATA III (6.0 Gb/s) for a traditional patter drive (HDD) as well as a custom PCIe x4 blade SSD connection.
Using a M.2 to SATA adapter won't help you here and a M.2 to Apple's custom connection still won't help you either.
Unlike the older models the PCIe NVMe interface driver Apple has only supports their SSD. I strongly recommend getting the SSD model (Apple PCIe) with as large an SSD as you can afford, then add in your own SATA drive if you want an internal deep storage option.
Frankly, while I understand your desire to have a unitized system (everything internally) Apple's design makes it hard to do that as they are using custom parts which is why using an external RAID'ed HD system makes a lot of sense once you get the SSD model. Then after the warranty period is over then think about adding the drive internally. Don't forget opening the new 'Thin Series' systems are a bear and if you don't use the correct tools and follow the IFIXIT guides to the letter you can damage the display which would be a very expensive Opps!
Lastly, don't forget Apple's SSD is the fastest out there! Apples custom design even beats Samsung's PCIe NVMe drive even though Samsung makes Apple's SSD drive! So you would be going backwards here in performance!