"In case of.." and "In the case of.." mean two different things so be careful. "In case of fire, take the stairs not the elevator" ('case' is a possibility) "In the case of Smith vs. Jones, the defendant pleaded guilty". ('case' is an actual thing) And yes, you can say "If possibilities permit" or "if possibilities allow" but it seems a bit odd. I would prefer to say "If the possibility exists, we will do it". You could, as an alternate, say "If circumstances permit..". that sounds natural to my ear and may be what you are trying to say anyway.