How Can I Add a Category Field to the BP Activity Plus Form


After searching the forums I haven’t been able to find how to alter a form defined by the BP Activity Plus plugin.

I would like to add a Category field without having to hack the plugin.

For example;

I create a plugin that will hook into that form and add a field that will be positioned based on its weight. Similar to Drupal.

Is this possible in WordPress and which Hook Filter should I use.

Thank you,