wp job manager custom column and filter

We have a child theme, and so we can add items into the functions file. This is a multisite set up. Using wp job manager we would like a custom dropdown list that will list all the sites within a multisite. When posting a job, from the primary site, this drop down is active and we can choose from any site. when posting a job from a child site, this dropdown is locked, and preselected to the current site. This option is NOT available / visible when applying for a job. A filter is also needed so this can be applied to the child sites (ie only display jobs that have been posted against that site.) The primary site will display all jobs. We have made a start of this, but ran out of time. As mentioned, if the code is provided, we can put this into the functions file rather than creating a plugin if this makes it cheaper.