Connect Kn-50 / Kn-51 Turbocell with MT?
No, that's not quite what was being dicussed here. SEC stands for "Super Ethernet Converter", and essentially acts like a wireless ethernet bridge - similar to the Senao CB3 units work. It was only available as a software load (under separate license) for WP-II/IIe, KN-100/105, and RG-1000 hardware (perhaps more, but that's about as far as my KN hardware experience goes).
As far as I know, a KN-50 (I'm not familiar with the KN-51) does not have the ability to be configured as an infrastructure mode wireless bridge, and are Turbocell only devices. As such, there is no chance that you will be able to connect a device speaking Turbocell to anything but a Turbocell base.
You *may* be able to find a firmware that could be loaded into KN-50 (and pre KN-50) units that brings them back to being just a wireless ethernet bridge, but I'm quite sure it would require some effort to get working (likely JTAG programming). I have a few old Maplenet "I-19" infrastructure mode devices that were identical in hardware to the "I-19" Turbocell devices that were shipped, so I know it it possible. It just may not be practical