Hello Imran,
You can duplicate your "old" structures, modify them and add the newly created ones that have the new financial dimension included. If you have already transactions recorded this should not be an issue as the changes will have an effect for future periods only.
Yet there are two things I would like to ask you to take an eye on: (a) I do not fully understand why you get the message that account structures overlap if you just add a new financial Dimension. This should actually not be an issue. So, please double check your existing account structures to check that everything is ok with their Setup. (b) Test the whole setup change first throughly before applying it in a live environment especially in regards to automatic transactions such as transactions created during inventory close, currency revaluation and alike to ensure that everything is fine.
All the best,
Ludwig