@Ronny **********
you cannot get a 30-days extension of the stay permit on top of a 90-days Non-Imm-O Retirement Visa. In order to achieve a 120 days stay in Thailand, you would need to exit Thailand before the 90-days stay permit expires, stay outside Thailand for a few days or up to one week, then re-enter Thailand visa-exempt. You will get stamped in for 60 days on a visa exempt entry ( guaranteed if your travel history to Thailand is not overly extensive)