Is it possible to add some user-defined Data Object based on existing cdc into Logical Node?
I mean to add new DO (PhvPhsA - phase to ground voltage for phase A) based on existing cdc (MV) into the Logical Node (MMXU).
In principle yes - it is possible to make extensions to the standard by using the appropriate namespace concept.
However, in your example, the data for the phase voltage, phase A already exists as:
MMXU.PhV.phsA of the CDC CMV
It is not compliant to the standard, to define new data objects as extensions to the standard for information that already exists in the standard. (Otherwise, the standard would not be a standard anymore)