df['sum'] = df['a'] + df['b'] # sum is a new column
df['Fruit Total']= df.iloc[:, -4:-1].sum(axis=1)
print (df)
Apples Bananas Grapes Kiwis Fruit Total
0 2.0 3.0 NaN 1.0 5.0
1 1.0 3.0 7.0 NaN 11.0
2 NaN NaN 2.0 3.0 2.0