I have extended my stay and want to ask for reimbursement of the rebooking fees
Sure. Travel plans change all the time, especially during fieldwork, pandemics and other situations. If you want the travel to be reimbursed, you can send the office or the cost center representative a short email to indicate that the plans have changed and you ask for an extension or a change of destination during the travel. A mail should suffice and is enough. If you extend for several weeks, you may be asked to fill out the application for approval again.
Usually, you only get reimbursed by the travel department for the exact travel that you applied for, but if the department later sees your mail/inquiry, there should not be any problem. Just note that expenses may rise when rebooking - these can also be reimbursed if you have plausible reasons :)