Sorry for the late response:

I considered implementing such a field in the first place, but what I’ve discovered is that recruiting companies don’t specify the salary. Some of them specify a wide range.
This is because they offer each applicant different conditions, depending on the applicant’s level and knowledge.

I think that the salary requirement is a UK requirement, as is probably the need to be able to give a job a reference number. I notice that in France they tend not to publish the salary in the newspaper adverts, where they do in the UK.

You could indeed have a go at adding a salary field in yourself.

I had a go as a result of this thread and have largely got it working.
http://www.jobberbase.com/community/discussion/76/adding-salary-and-expires-field/