All the entires of the sublist should be tagged and later the end tag of item 2 should be inserted. Also after finishing the sublist, tagging for the main list should continue ...

I have a feeling that the OP wants this:

<listitem>List item 1</listitem>
<listitem>List item 2
  <listitem>List item a</listitem>
  <listitem>List item b</listitem>
  <listitem>List item c</listitem>
</listitem>
<listitem>List item 3</listitem>
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(indentation added for emphasis.) Is this correct?