As @brettdj already pointed out with his link to cpearson.com/excel/vbe.aspx , you can programmatically change to code of a VBA module using the VBA Extensibility library! To use it, select the library in the VBA editor Tools->References. Note that you need to also change the options in your Trust center and select: Excel Options->Trust Center->Trust Center Settings->Macro Settings->Trust access to the VBA project object model
Then something like the following code should do the job:
Private mCodeMod As VBIDE.CodeModule
Sub UpdateModule()
Const cStrModuleName As String = "Source"
Dim VBProj As VBIDE.VBProject
Dim VBComp As VBIDE.VBComponent
Set VBProj = Workbooks("___YourWorkbook__").VBProject
'Delete the module
VBProj.VBComponents.Remove VBProj.VBComponents(cStrModuleName)
'Add module
Set VBComp = VBProj.VBComponents.Add(vbext_ct_StdModule)
VBComp.Name = cStrModuleName
Set mCodeMod = VBComp.CodeModule
'Add procedure header and start
InsertLine "Public Function GetSource() As String"
InsertLine "Dim s As String", 1
InsertLine ""
'Add text
InsertText ThisWorkbook.Worksheets("Sourcetext") _
.Range("___YourRange___")
'Finalize procedure
InsertLine "GetSource = s", 1
InsertLine "End Function"
End Sub
Private Sub InsertLine(strLine As String, _
Optional IndentationLevel As Integer = 0)
mCodeMod.InsertLines _
mCodeMod.CountOfLines + 1, _
Space(IndentationLevel * 4) & strLine
End Sub
Private Sub InsertText(rngSource As Range)
Dim rng As Range
Dim strCell As String, strText As String
Dim i As Integer
Const cLineLength = 60
For Each rng In rngSource.Cells
strCell = rng.Value
For i = 0 To Len(strCell) \ cLineLength
strText = Mid(strCell, i * cLineLength, cLineLength)
strText = Replace(strText, """", """""")
InsertLine "s = s & """ & strText & """", 1
Next i
Next rng
End Sub