UPDATE:
I've written the code below, which more or lets lets me do what I was hoping to, for two clickboxes. I can keep this up for the third, fourth, etc. but would love any thoughts on whether there's a more elegant way to do so -- particularly with the various conditional statements. (I'll probably have 20 boxes in total.) Thanks again!
Private Sub cmdAddToDocument_Click()
Application.ScreenUpdating = False
Dim strNewText As String
Dim text1 As String
Dim text2 As String
text1 = "Random string of text."
text2 = "Another random string of text."
If cbx1 = True Then strNewText = text1
If cbx2 = True Then strNewText = strNewText & vbNewLine & text2
With ActiveDocument
Selection.Text = strNewText
End With
Application.ScreenUpdating = True
Unload Me
End Sub