Well.. I'd go about getting their permission first whomever it is that made the island....
if it is one that is a user made island, then you should be able to de-pbo it using a utility (you can find a couple in the editor's depot section of this site)
and if it is a BIS one, you already have the wrp file in your Worlds folder...
except for Nogova which is slightly more complicated to go about getting into ;D
You would then have to have the program Wrp Edit to do any editing of the .wrp file its self..
There are a bunch of tutorials out there regarding WrpEdit and that is where i suggest you start looking