A warp storm in the W40k universe sucks in part of a Hive Fleet and drops it into the B5 universe. It immediately attacks a member of the Alliance (some mundane world). Can the Alliance handle the threat? Here's a link for those not sure what a tyranid is: http://wh40k.lexicanum.com/wiki/Tyranid
ISA doesn't stand a chance, the Tyranids would utterly annihilate anything the B5 universe could throw at it B5 ships tend to lack shields and WH40K firepower levels are much higher than B5.
It wouldn't even be a fair fight, the Tyranids would win easily. The ISA would get demolished in a fight against the Space Marine Chapters.
I guess it depends on how much "part" of a Hive fleet is and where it gets dropped off. If it is a very small part they get hit early in space the Alliance might be able to deal with it if they take the threat extremely seriously and go for the kill immediately. If the Hive Fleet has time to do some leg work, get to know this universe its in and strips a few planets along the way they would most likely steamroll the ISA.