En fait, voilà, il me restait une demi-heure ...
Sub JSON()
Sheets("Liste écoles - Carte").Select
Close #1
Open ThisWorkbook.Path & "\" & ActiveSheet.Name & ".json" For Output As #1
For Each cel In [DEBUT]
Print #1, cel.Value
Next
For i = 2 To Cells(Rows.Count, "A").End(xlUp).Row
For Each cel In [MILIEU]
If cel.Value Like "*COL*" Then
col = Mid(cel.Value, InStr(cel.Value, "COL") + 3, 2)
txt = Replace(cel.Value, "COL" & col, Cells(i, col * 1).Value)
Print #1, txt
Else
Print #1, cel.Value
End If
Next
If i <> Cells(Rows.Count, "A").End(xlUp).Row Then Print #1, ","
Next
For Each cel In [FIN]
Print #1, cel.Value
Next
Close #1
End Sub