Great question. You can add more options or change the existing ones for the staff section. Since you don’t need the Facebook and Twitter ones, it’s a bit easier to edit than add anyway. Here’s where you’ll need to go.
1) Go to the appearance section and select edit to open up the theme editor (or use the editor of your choice).
2) From the list of files on your right, choose fields.php and look down to line 45 for this spot:
Twitter Username
and change it to say LinkedIn URL and save your changes.
3) You’ll need to make the following change on the page-staff.php, single-staff.php and taxonomy-staff-group.php pages. Look for where you see this:
<?php if(get_post_meta($post->ID, "_twitter", $single = true) != ""){ ?>
<span class="twitter-link"><a href="http://www.twitter.com/<?php echo get_post_meta($post->ID, "_twitter", TRUE); ?>"> Twitter </a></span>
<?php } else { ?>
<?php } ?>
and replace the Twitter with LinkedIn and save your changes.
Now you can use the spot that previously was for Twitter for your LinkedIN profile URL. If you have any questions, let me know.