numpy array merge

[Nathan]

Is there an easy way to merge two numpy arrays with different rank
sizes (terminology?).  I want to make a single array by concatenating
two arrays along a given direction and filling the excess cells with a
dummy variable.  numpy concatenate works well as long as the two
arrays have the same dimension, but I want to do this on two array
with a matching dimension size.  An example:

import numpy as np
a = np.array([[1,2,3],[4,5,6]])
b = np.array([[1,2],[3,4],[5,6]])
np.concatenate((a,a),axis=1)  #This works fine, but isn't what I need.
Out:  array([[1, 2, 3, 1, 2, 3],
       [4, 5, 6, 4, 5, 6]])
np.concatenate((a,b),axis=1)  #This doesn't work, but this is what I
need.

I want to fill the third row of array "a" with a dummy variable (99999
or NaN) and concatenate with "b" to make:
[1,2,3,1,2]
[4,5,6,4,5]
[99999,99999,99999,5,6]

This just seems like it would be relatively common.  So I thought I'd
check if I'm missing something before I go write the logic.  Thanks!