Add button in the Undetectable Types tab in the window.

Use Associate Format settings to assign a specific plug-in module and attributes to a file type or filename extension.

Associate Format Settings
Setting Description
Extension Sets the file type extension to be association to a format, such as vox, snd, raw. Do not insert a leading period.
File type Sets the plug-in module to assign to the file type.
Attributes Sets the specific (channels, quality, rate, etc.) to assign to the file type.
Rate Sets the default sampling rate for the file type, if not already assigned in Attributes.

Some plug-in modules allow custom attributes to be set. If so, the Custom button will be enabled and can be used to set those attributes.