There are a few ways of doing this.  If you are a Daz Studio user, the Morph Loader Pro tool makes this easy.  If you don't use DS, you can do it manually in Poser.  Just create your morph in you modelling ap, then load the morph into your clothes in Poser.  Use a text editor on your clothing cr2 to add in the code for making the JCm automatic.  Here is an example of the code for M4's collar bend JCM from my Immortal Ronin shirt.  I only needed the JCm to activate when the collar was bent up, so that's why the minimum value is set at zero.