Simplest way is to undock from ISS and do an orbital plane alignment burn to shift your plane over to MIR. It will take a lot of fuel because they are in such different planes. I believe that someone once wrote a scenario that challenged you to dock/land/rendevous with a number of objects/locations and that there was a method whereby someone used an orbit around the moon to change alignment for a MIR/ISS transfer, but I can't be sure about that.