hook alter for _simplenews_subscription_manager_form($subscription)

Hello, I'm trying to modify the form (the form block actually) to hide the subscribe / unsubscribe radio buttons and instead use an input hidden type with a value = "Subscribe".

I found the function called is _simplenews_subscription_manager_form($subscription) from other functions. Is there a way I could just change the form without making a custom function, then call the new custom functions from everywhere where _simplenews_subscription_manager_form($subscription) is executed?

I didn't found any post that really covered it, but if there is one for Drupal 6, that would be great.

Thanks!