Drupal Webforms: Can I use the Conditions to control how the contacts are filtered?
We ran into the same issue and didn't want to customise the webform module as it is heavily relied on for a lot of data input and needs to be kept secure and easy to maintain.

So we built a Drupal view displaying the Volunteers based on their relationship to the Staff member (I'm assuming they are related in CiviCRM). When the Staff Member visits the page, it can restricted to show only those the have an active relationship to using a contextual filter to the Drupal User.

This post should help if you've not done this before: How to create a Drupal View with multiple CiviCRM relationships?

We then created a link (using the views Global Text field) to the webform to prepopulate the Volutneer as cid2 or contact 2 (assuming the staff member is cid1) which looks something like this: 
myorganization.org/node/1?cid2=[id] where [id] is the id of the volunteer.