You can use pd.DateOffset:
test[1].index + pd.DateOffset(hours=16)
pd.DateOffset accepts the same keyword arguments as dateutil.relativedelta.
The problem you encountered was due to this bug which has been fixed in Pandas version 0.14.1:
In [242]: pd.to_timedelta(16, unit="h")
Out[242]: numpy.timedelta64(16,'ns')
If you upgrade, your original code should work.