This can only be done by editing the xml (I think, I'm not sure if it still works).
what this option do? (special\code)
I don't see any reference into the manual...
So i absolutly need an helper block like this... (?)
Yeah, I guess so.
Not necessarily. There is a primitive block object [myself v] whose input is a list of all the sprites available in the project.
You can replace the block helper with it.
Since you want your variadic input to only accept object block, in the "edit input name" dialog, select the object and then multiple inputs.
It looks like this: