import pandas as pd
import datetime

data = pd.DataFrame([['G1', '2018010101'], ['G2', '2018011101']], columns=['a', 'b'])

def convertDateStr(value):
    return datetime.datetime.strptime(value, "%Y%m%d%H").strftime("%Y-%m-%d %H:%M:%S.000")

data.b = data.b.apply(convertDateStr)

print(data)