Impeachment is for crimes committed while in office.
Rosenstein made sure Mueller could go after process crimes.Mueller can only write a report making recommendations. Those would include high crime or misdemeanors:
“(ii) any matters that arose or may arise directly from the investigation; and...
(c) If the Special Counsel believes it is necessary and appropriate, the Special Counsel is
authorized to prosecute federal crimes arising from the investigation of these matters.”
https://assets.documentcloud.org/documents/3726408/Rosenstein-letter-appointing-Mueller-special.pdf