Is there a way to create a custom parameter in the fixture editor?

Political correct answer ro custom parameters is No.
Don't try it, it is not comfortable.

With the different DMX adresses in one parameter... i think i saw it once in a fixture.
I will try to remember.

As I suspected.

All I want to do is to create parameter names "Forward", "Backwards", "F.Speed" and "B.Speed".

I can already do the non-intens parameters using the DMX values ranges.

For the Speed, you can use "Global MSpeed" & "Global MSpeed2" in >Form< Or if possible only use one and have them set to +/-

And for the direction...

---

All different.
You have a "Master" of Forward or Backwards.
If you go Forward, than only the Forward speed (parameter Global MSpeed for example) will increase in positive direction.
If Backwards is selected, the parameter will set up the other value to 0 and move the negative speed (Backward) up.

You likely will go with some virtual parameters.
For Motor-direction you could use "Effect Direction" as a virtual parameter.

Let me wrap my head around that... 

For the Speed, you can use "Global MSpeed" & "Global MSpeed2" in >Form< Or if possible only use one and have them set to +/-

And for the direction...

---

All different.
You have a "Master" of Forward or Backwards.
If you go Forward, than only the Forward speed (parameter Global MSpeed for example) will increase in positive direction.
If Backwards is selected, the parameter will set up the other value to 0 and move the negative speed (Backward) up.

You likely will go with some virtual parameters.
For Motor-direction you could use "Effect Direction" as a virtual parameter.

Let me wrap my head around that...