In short, no. You should not microwave an envelope open.
While it is technically possible to microwave an envelope open, it is not recommended. Envelopes are made of paper and are not meant to be microwaved. When microwaved, the paper can catch fire or release harmful chemicals.
If you need to open an envelope, it is best to do so manually. Use a sharp knife or scissors to carefully slit open the envelope. Be careful not to damage the contents of the envelope.