This should get you started: Using VBA in your own Excel workbook, have it prompt the user for the filename of their data file, then just copy that fixed range into your target workbook (that could be either the same workbook as your macro enabled one, or a third workbook). Here's a quick vba example of how that works:
' Get customer workbook...
Dim customerBook As Workbook
Dim filter As String
Dim caption As String
Dim customerFilename As String
Dim customerWorkbook As Workbook
Dim targetWorkbook As Workbook
' make weak assumption that active workbook is the target
Set targetWorkbook = Application.ActiveWorkbook
' get the customer workbook
filter = "Text files (*.xlsx),*.xlsx"
caption = "Please Select an input file "
customerFilename = Application.GetOpenFilename(filter, , caption)
Set customerWorkbook = Application.Workbooks.Open(customerFilename)
' assume range is A1 - C10 in sheet1
' copy data from customer to target workbook
Dim targetSheet As Worksheet
Set targetSheet = targetWorkbook.Worksheets(1)
Dim sourceSheet As Worksheet
Set sourceSheet = customerWorkbook.Worksheets(1)
targetSheet.Range("A1", "C10").Value = sourceSheet.Range("A1", "C10").Value
' Close customer workbook
customerWorkbook.Close