IBM has one and only one recommended way to increase the size of a PDS directory: copy the PDS to another PDS which has the larger directory.
If your site has the PDS software from the CBT tape at http://www.cbttape.org
then you can use it to increase the size of a PDS directory without having to copy it to another PDS. There are also vendor products that can do this. If your site is limited to the IBM utilities, however, there is ABSOLUTELY no way to dynamically change the size of a PDS directory using IBM software.