Most (if not all at least core developed addons, and a large number of community ones too) have support for stuff like that. If it's not documented, you can check the "Properties" tab to see if there's any hints there, or look through the first bit of the PHP for pieces of code like this:
$something = $modx->getOption('something',$scriptProperties,..);
That indicates it is looking for a "something" property, and, when named properly, that should be reassurance something is possible after all!
Oh, and of course make sure to post your findings of undocumented properties at
http://bugs.modx.com/projects/docs/issues so someone with access to it can update it, and save someone else the trouble next time