Hello Thomas,
Look in the API help for: Custom Parameter Groups, and you will find this code:
it might help you. (This is API only!)
Public Sub CreateParametersAndGroup()
' Create a new Part document.
Dim oPartDoc As PartDocument
Set oPartDoc = ThisApplication.Documents.Add(kPartDocumentObject, _
ThisApplication.FileManager.GetTemplateFile(kPartDocumentObject))
' Set a reference to the compdef.
Dim oCompDef As PartComponentDefinition
Set oCompDef = oPartDoc.ComponentDefinition
' Create a model parameter
Dim oModelParam As ModelParameter
Set oModelParam = oCompDef.Parameters.ModelParameters.AddByValue(2, kCentimeterLengthUnits)
' Create a reference parameter
Dim oReferenceParam As ReferenceParameter
Set oReferenceParam = oCompDef.Parameters.ReferenceParameters.AddByValue(4, kCentimeterLengthUnits)
' Create a user parameter
Dim oUserParam As UserParameter
Set oUserParam = oCompDef.Parameters.UserParameters.AddByValue("length", 6, kCentimeterLengthUnits)
' Create a new custom parameter group
Dim oCustomParamGroup As CustomParameterGroup
Set oCustomParamGroup = oCompDef.Parameters.CustomParameterGroups.Add("Custom Group", "CustomGroup1")
' Add the created parameters to this group
' Note that adding the parameters to the custom group
' does not remove it from the original group.
Call oCustomParamGroup.Add(oModelParam)
Call oCustomParamGroup.Add(oReferenceParam)
Call oCustomParamGroup.Add(oUserParam)
End Sub
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Kudo's are also appreciated

Succes on your project, and have a nice day
Herm Jan