Think you will be lucky to get the extra stamp to stay. Forgetting to go to immigration is no excuse you don't forget important things like that if you wish to stay in the country. You need to go to immigration ASAP and don't have more overstay.
I applied 3 weeks ago and they told gf be 10 weeks because of backlog processing Ukraine visas. Don't book a flight till visa is issued unless its a ticket you can change dates without charge.