set length user parameter (untested)

example.vb

Sub Main 
  Dim asmDoc As AssemblyDocument = ThisApplication.ActiveDocument 
  
  'Iterate through all of the referenced documents
  For Each doc As Document In asmDoc.AllReferencedDocuments 
      'update the components 
  If doc.DocumentType = DocumentTypeEnum.kPartDocumentObject Then
    AddLengthParameter(doc) 
  End If
  Next 
End Sub

Sub AddLengthParameter(oDoc As PartDocument)
  'Get the user parameters collection. 
  Dim userParams As UserParameters = oDoc.ComponentDefinition.Parameters.UserParameters 

  Dim param As Parameter 
  'Set/create parameter
  Try
    param = userParams.Item("Length") 
    param.Expression = "B_L"
  Catch
    'The parameter doesn't exist so add it. 
    param = userParams.AddByExpression("Length", "B_L", UnitsTypeEnum.kDefaultDisplayLengthUnits) 
  End Try
  param.ExposedAsProperty = true
End Sub