osx can't out of the box write ntfs (windows file system) there are 3th party programs which make it possible
windows can't out of the write apple osx filesystem there are 3th party programs which make it possible
changing size of a partition (a harddisk devices into several portions is called partitions)
is no simple task there are tools which can do it but they are often expensive like the tool partition magic not even sure if it could do it though
easiests way would be take a backup of windows delete bootcamp parition for windows
setup a new bootcamp partition with more space
install windows
restore the backup
it's been the way for as long as people been dual booting with windows and linux and os2 and the likes