Adding new instructions to the processor lx106

Question

In the xtensa documentation there is a paragraph saying that it is possible to 'burn' new instructions in the processor itself using a sort of C language. How to create new instruction in an assembly source program or how to link this new instruction to an assembly source. For example there is no push instruction and i want to create one. Please note that i am not asking about how to do a push in a stack which i already know.