In CATIA it is not possible to add the symbol envelope direct to a diameter.

Here there is a workaround to add it if the dimension:

• is not a diameter or a radius
  Properties ➞ Dimension Texts ➞ Main Value ➞ click on the symbol
• is a diameter or a radius
  Properties ➞ Dimension Texts ➞ Prefix Suffix ➞ choose the group of symbols (first icon) ➞ Main Value ➞ click on the symbol

If you want to see step by step how to do it, please follow the instruction which is in the document below.

 Envelope Symbol