This is the code I came up with after some time playing around.
So I managed to insert new operators into the database sheet and sort them like I wanted, I had a problem sorting because the next column was beeing sorted as well even tough I'd only selected the correct range! The problem solver here was leaving the next column, "B", alone and voilá 
Furthermore i added some more code to avoid numeric and nullstring entries but i think it needs some more work.
Afterwards I tried to assign the rowsource to the listbox but I got an error, dunno what I'm doing wrong there some help would be nice 
You can check all of this in the attached file, in the Index sheet press Utilities > System > Operators
Cheers!
Code:
Private Sub btnInsert_Click()
Dim wb As Workbook
Dim wsDatabase As Worksheet
Dim rngOperators As Range
Dim lastRow As Long
Set wb = Workbooks("test.xlsm")
Set wsDatabase = wb.Sheets("database")
lastRow = wsDatabase.Range("a" & Rows.count).End(xlUp).Row
Set rngOperators = wsDatabase.Range("a2", "a" & lastRow)
Dim text As String
Dim insert As Integer
Dim verify As Integer
verify = 0
insert = MsgBox("Insert?", vbYesNo + vbExclamation, "v1.0")
text = usfOperators.txtOperator.Value
If insert = vbYes And IsNumeric(usfOperators.txtOperator.Value) Then
MsgBox "Insert a name not a number!", vbCritical, "v1.0"
usfOperators.txtOperator.Value = ""
usfOperators.txtOperator.SetFocus
verify = verify + 1
Exit Sub
ElseIf insert = vbYes And text = vbNullString Or text = " " Then _
MsgBox "Field is empty!", vbCritical, "v1.0"
usfOperators.txtOperator.Value = ""
usfOperators.txtOperator.SetFocus
verify = verify + 1
Exit Sub
End If
If verify = 0 Then
With Sheets("database")
Cells(lastRow + 1, 1) = text
MsgBox "New operator was added!", vbInformation, "v1.0"
End With
Selection.Sort Key1:=Range("a2"), Order1:=xlAscending, Header:=xlGuess, OrderCustom:=1, MatchCase:= _
False, Orientation:=xlTopToBottom, DataOption1:=xlSortNormal
usfOperators.txtOperator.Value = ""
usfOperators.txtOperator.SetFocus
End If
End Sub