As it turns out, you cannot define supporting actors in a matrix. The SupportingActors keyword is only valid in the context of a use case. Essentially, the $matrix keyword is another version of $list, that does not allow you to set a context other than the default project level.
You can use a nested repeat loop to return the same information. A quick report example:
$repeatUseCases where SupportingActors exist
Supporting Actors for UseCase $ID $Name
$repeatSupportingActors
$ID $Name
$endrepeatSupportingActors
$endrepeatUseCases