if you already have a post grad work permit..i dont think you need an LMO...because all PGWP are open permits so you can work for any employer..please check the permit and see what it says on it....
LMO doesnot depend on PGWP..Its depends on availability of skills in the area. If u have exp under PGWP and then i am sure you will get positive LMO because of your Canadian experience.
Correct me if I am wrong but I think getting a positive LMO totally depends upon your employer and the type of job that is going to be created for a foreign national.
Once your employer is approved of the job and gets the LMO , you can apply for the work permit using that LMO.