Yes, customers can purchase Extra Seat (EXST) if he/she wishes to keep adjacent seat not occupied by another passenger. It should be customer’s choice whether they buy an adjacent seat or the whole Quad.
Please note, that the only seats that will offer additional space at a seat / bed level are the AFT
facing seats, EF.
Visit this link for more details on Qsuite https://www.qatarairways.com/en/onboard/qsuite.html