Thanks for your detailed answer, and that you took so much time to help me! :)

Some questions to the process. Let's say I have a V3 character that has only the EyeDepth Morph set to 1, and I want to fit my Eyes to this character. I'd like to point out that it doesn't concern about one specific character, but I want to fit my eyes to any character that is available at Rero marketplace ;)

Question about the process of creating

I make my morph in an external program and load it in Poser with selecting each eye separatly with "Object > Load Morph target". These morphs R+L I call "EyeDepth" like it's called in the Dials of V3. I save the new CR2 to the library. 

Now I open my new CR2 in a text editor and edit it in the following way:

targetGeom EyeDepth  
{
name EyeDepth
initValue 0
hidden 0
forceLimits 1
min -100000
max 100000
trackingScale 0.02
keys
{
static  0
k  0  0
}
interpStyleLocked 0
valueOpDeltaAdd
Figure
head
PBMCC_20 (???)
deltaAddDelta 1.000000
numbDeltas 2500
useBinaryMorph 1
blendType 0
}

Should I insert in the green line the name of the V3 morph "EyeDepth"?

Then I get some lines above and change the code from this:
actor head:1
{

}

to this:

actor head:1
{
channels
{
groups
{
groupNode MyEyes
{
parmNode EyeDepth
}
}
targetGeom EyeDepth
{
name EyeDepth
hidden 0
keys
{
k 0 0 
}
}
}

How do I determine the right groupNode? I never named the group "MyEyes" before. How does Poser know which body parts I want to be affected?
Somewhere I must have a mistake. When I try the load the new CR2, Poser shows "No figure" and "No actor". With the original CR2 it works, the modified must have a mistake...

**Question about process of using

**At the moment I created the CR2 and a hide pose for the eyes of V3.
If I understood you right, the process would be to load V3, apply the hide pose, apply the character pose and select V3. With the selected V3, load the Eyes.cr2 and conform it to V3.
Am I right?