Changing a node value for a variable

I have a question:

Since a variable may contain a node set, is it possible to modify the value
of one node in the variables nodes.

Example:
<foo>
  <data1>Tom</data1>
  <data2>Jones</data2>
</foo>

to

<foo>
  <data1>Bridget</data1>
  <data2>Jones</data2>
</foo>

or is it possible to add a new child node?

Example:
<foo>
  <data1>Tom</data1>
  <data2>Jones</data2>
</foo>

to

<foo>
  <data1>Tom</data1>
  <data2>Jones</data2>
  <data3>Bridget</data3>
</foo>

Thanks,
-Jeff Harman

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