You could use a formula for your mergefield, coded along the lines of:
{=MOD(INT({MERGEFIELD MyField}/10),100) \# 00}
or:
{=MOD(INT(«MyField»/10),100) \# 00}
where 'MyField' is your field name.

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. Likewise, you can't type or copy & paste the chevrons (i.e. '« »') - they're part of the actual mergefields, which you can insert from the 'Insert Merge Field' dropdown.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]