As stated above, you can disable stages in config.lua in order to allow monsters to give experience forever.
If you want to use stages, you can simply omit the maxlevel modifier on the last multiplier stage.
This will provide the same allowance on experience as the previous suggestion, where monsters will give experience forever without a level limit.
XML:
<?xml version="1.0" encoding="UTF-8"?>
<stages>
<world id="0" multiplier="1">
<stage minlevel="1" maxlevel="250" multiplier="10"/>
<stage minlevel="251" maxlevel="499" multiplier="5"/>
<stage minlevel="500" multiplier="1"/>
</world>
</stages>