With protected forms ruled out, you cannot do it automatically without macros, so you would need to save the document as either a macro enabled document or template and the users would have to allow the macro to run.
For Word 2013 or 2010 (Word 2007 will require additional code, and it will not work in earlier versions which do not support content controls)
Insert a plain text content control at each location, name the first SQFT and the second SQM. Set the default texts of each to 0 and check the property to disable deletion. In the second field only check the 'Contents cannot be edited check box' property
In the ThisDocument module of the document/template add the following code which will calculate to two decimal places the value of sq ft in sq m.
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal CC As ContentControl, Cancel As Boolean)
Dim oCC As ContentControl
Select Case CC.Title
Case "SQFT"
'Validate/format data entered in variable content control.
If Not IsNumeric(CC.Range.Text) Then
Cancel = True
Beep
CC.Range.Text = "0"
With ActiveDocument.SelectContentControlsByTitle("SQM").Item(1)
.LockContents = False
.Range.Text = "0.00"
.LockContents = True
End With
CC.Range.Select
Exit Sub
Else
CC.Range.Text = Format(Val(CC.Range.Text))
End If
'Update content control contents.
Set oCC = ActiveDocument.SelectContentControlsByTitle("SQM").Item(1)
With oCC
.LockContents = False
.Range.Text = Format(Val(ActiveDocument.SelectContentControlsByTitle("SQFT").Item(1).Range.Text * 0.09290304), "0.00")
.LockContents = True
End With
End Select
Set oCC = Nothing
lbl_Exit:
Exit Sub
End Sub