Hello,
There's really no truly straight-forward answer to the question you have given.
Generally speaking, adding select (or other more sophisticated field types) is not possible via administration panel. Nor the use of administration panel is needed or even recommended.
To create new multiselect field it's best to create custom module of your own (preferably in modules/Custom/) and in instllation precedure execute code that will add this field and set the criteria (filtering) method to your function, in your module, in *Common part of it.
Now you mentioned you have already have a table with contacts that should be placed in the select - that's not how we usually work with contacts in epesi. Usually, we resort to creating a Contact Group that will represent the kind of people we want in our select and then construct criteria as
array('group'=>'salepeople')
This way we do not need to manipulate external tables to manage those selections and we avoid data redundancy.
All of this, however, requires a bit of php coding. If you're up to it, best check our tutorials on
http://www.epesi.org/index.php?title=Epesi_tutorial
Especially sections:
http://www.epesi.org/index.php?title=Cr ... dule_files
http://www.epesi.org/index.php?title=Ut ... stallation
http://www.epesi.org/index.php?title=Ut ... ield_Types
And finally:
http://www.epesi.org/index.php?title=Install_new_module
This should give you most of the intormation you need to create such select field.
Kind regards,
Arek