It is possible - you can change anything you like in linuxcnc.
Depending on how much you wish to change,
you need to know about python, pygtk and GLADE.
There is also Gscreen which caters more to customisations which may or may not be appropriate depending on what kind of changes you want and how independent to linuxcnc updates you hope to be.
There is no detailed manual for this.
There is a very basic overview here:
www.linuxcnc.org/docs/2.7/html/gui/gscreen.html
Gmoccapy uses similar techniques.
Chris M