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You can add the VBA below to your document, and then use the function like this:

=CONCATENATEIF(A1:A8, "a", B1:B8, "_")

VBA CODE:

Function ConcatenateIf(CriteriaRange As Range, Condition As Variant, _
        ConcatenateRange As Range, Optional Separator As String = ",") As Variant
    Dim i As Long
    Dim strResult As String
    On Error GoTo ErrHandler
    If CriteriaRange.Count <> ConcatenateRange.Count Then
        ConcatenateIf = CVErr(xlErrRef)
        Exit Function
    End If
    For i = 1 To CriteriaRange.Count
        If CriteriaRange.Cells(i).Value = Condition Then
            strResult = strResult & Separator & ConcatenateRange.Cells(i).Value
        End If
    Next i
    If strResult <> "" Then
        strResult = Mid(strResult, Len(Separator) + 1)
    End If
    ConcatenateIf = strResult
    Exit Function
ErrHandler:
    ConcatenateIf = CVErr(xlErrValue)
End Function