So something along these lines?
<abs contexttype="opengear">
<meta>
<params>
<param access="1" maxlength="0" name="User Input" oid="params.userinput" stateless="true" type="STRING" value="" widget="default"/>
<param access="1" maxlength="0" name="XPression Key" oid="params.xpressionkey" stateless="true" type="STRING" value="" widget="label"/>
<param access="1" maxlength="0" name="Excel Key" oid="params.excelkey" stateless="true" type="STRING" value="" widget="label"/>
</params>
</meta>
<param expand="true" height="91" left="12" oid="params.userinput" runtasksonload="true" showlabel="false" style="size:Big;txt-align:center;" top="12" width="266">
<task tasktype="ogscript">var userInputValue = params.getValue('params.userinput', 0);
if (userInputValue == '90')
{
params.setValue('params.xpressionkey', 0, '0');
params.setValue('params.excelkey', 0, '90');
}
else if (userInputValue == '99')
{
params.setValue('params.xpressionkey', 0, '00');
params.setValue('params.excelkey', 0, '99');
}
else
{
params.setValue('params.xpressionkey', 0, userInputValue);
params.setValue('params.excelkey', 0, userInputValue);
}</task>
</param>
<simplegrid height="171" hspace="5" left="12" rows="1" top="116" width="388">
<param expand="true" oid="params.xpressionkey" showlabel="false" style="size:Biggest;txt-align:center;font:bold;bg#dark;bdr:etched;"/>
<param expand="true" oid="params.excelkey" showlabel="false" style="size:Biggest;txt-align:center;font:bold;bg#dark;bdr:etched;"/>
</simplegrid>
<button buttontype="push" height="97" left="281" name="Go!" top="11" width="111"/>
</abs>
#DashBoard