title: Add data slice (i.e. n-by-n matrix) to multidimensional matrix in R
body: <p>As per the title, is there a way to append a data slice, which is an n-by-n matrix to an existing N-dimensional matrix in R?</p>
  
  <p>For example, I have the following:</p>
  
  <pre><code>one &lt;- array(1, dim = c(3, 3))
  two &lt;- array(2, dim = c(3, 3))
  three &lt;- array(3, dim = c(6, 6))
  </code></pre>
  
  <p>Which I would then like to have transformed into a 6x6x3 matrix that I can work with, which would look like the following:</p>
  
  <pre><code>[[1]]
       [,1] [,2] [,3] [,4] [,5] [,6]
  [1,]    1    1    1   NA   NA   NA
  [2,]    1    1    1   NA   NA   NA
  [3,]    1    1    1   NA   NA   NA
  [4,]   NA   NA   NA   NA   NA   NA
  [5,]   NA   NA   NA   NA   NA   NA
  [6,]   NA   NA   NA   NA   NA   NA
  
  [[2]]
       [,1] [,2] [,3] [,4] [,5] [,6]
  [1,]    2    2    2   NA   NA   NA
  [2,]    2    2    2   NA   NA   NA
  [3,]    2    2    2   NA   NA   NA
  [4,]   NA   NA   NA   NA   NA   NA
  [5,]   NA   NA   NA   NA   NA   NA
  [6,]   NA   NA   NA   NA   NA   NA
  
  [[3]]
       [,1] [,2] [,3] [,4] [,5] [,6]
  [1,]    3    3    3    3    3    3
  [2,]    3    3    3    3    3    3
  [3,]    3    3    3    3    3    3
  [4,]    3    3    3    3    3    3
  [5,]    3    3    3    3    3    3
  [6,]    3    3    3    3    3    3
  </code></pre>
  
  <p>I know how to do this via my own code, so I'm more interested in if there is an existing library function that supports this. </p>