Adding rows with different column names:

import pandas as pd

a = {'A': 10, 'B': 20}
b = {'B': 30, 'C': 40, 'D': 50}

df1 = pd.DataFrame(a, index=[0])
df2 = pd.DataFrame(b, index=[1])

df = pd.DataFrame()
df = df.append(df1)
df = df.append(df2).fillna(0)

print(df)

C:\pandas>python example37.py
      A   B     C     D
0  10.0  20   0.0   0.0
1   0.0  30  40.0  50.0