There are a couple of issues to consider.
1. The signal connection via a Trainer port or similar
2. Channel ordering for the sticks
On the Trainer port...
The MX18 has a DSC port which appears to be a PPM trainer port which can be configured to be in (Master) or out (Student).
The manual
http://www.towerhobbies.com/manuals/gpn/gpnj1005_manual.pdf, is pretty incomprehensible. Search for DSC and also look at pages 50,51 for trainer set up.
Not sure why they call it something else and then don't define what DSC means but....whatever. The MZ18 also supports a wireless trainer feature but you would need another Graupner to use that.
From this video which is less helpful than it might have been...
https://www.youtube.com/watch?v=BjYulEiHRAM
...it appears you can attach an MZ12 as a student to a Taranis with a mono 3.5mm audio cable which indicates that the CPPM out is compatible with that TX. Since the Taranis can Master a Spektrum DX6i also it seems there is a good chance that the Trainer port is compatible between the Graupner and Spektrum. Spektrum requires a mono jack to ensure a good connection. No clear what the DSC port requires but it seems a mono audio jack of some kind works.
So that leaves the channel order...
A DX6i uses TAER and Graupner uses... well I can't tell from a quick reading of the manual. It's very dense and obscure. A case of lots of pictures obscure what a few words would say. I suspect you can map the channels as you want which means you would be able set up your Graupner receiver as TAER to agree with the Spektrum student radio but I can't really confirm that without more study than I'm prepared to do.
So all I can say is it probably works but if you are set on the Graupner with Spektrum you should dig a bit deeper.
