For Python2.7
You can format it into seconds and then multiply by 1000 to convert to millisecond.
from datetime import datetime
d = datetime.strptime("20.12.2016 09:38:42,76", "%d.%m.%Y %H:%M:%S,%f").strftime('%s')
d_in_ms = int(d)*1000
print(d_in_ms)
print(datetime.fromtimestamp(float(d)))
Output:
1482206922000
2016-12-20 09:38:42