Somehow you can. If you have called up someone for instance #121 , and you have saved the contact named as A in your contact list. Go to phone app , tap on the contact icon (extreme right towards the name ) , edit and then edit the #.
Yes.copy the dialed number by holding and paste it to dialer.
If want to add something before number like 0 or 91 then press 0 then paste your copied number.