[Admin]

Not really.

You will need to edit two things:
memberlist.php file
and
memberlistsearch template

For the file, you should add something like this:

    if ($customfieldname!="") {
      $condition.=" AND userfield.field5 LIKE '%".addslashes($customfieldname)."%' ";
    }

(but change the red parts to your needs)
somewhere in this block:

    if ($icq!="") {
      $condition.=" AND icq LIKE '%".addslashes($icq)."%' ";
    }
    if ($aim!="") {
      $condition.=" AND aim LIKE '%".addslashes($aim)."%' ";
    }

(lots of stuff like that there, just place it between them, doesn't matter where)

Then you will need to add an input box to the template, and make the name of it customfieldname, so it'll match the code you added earlier.

Not really hard, but takes a bit time.