If the new locations are being setup from scratch, you make sure your carrier provides you with number ranges that don't conflict in the last 4 digits with your existing ones.
If the new locations are already setup you could change their number ranges and extensions so there is no conflict. Otherwise, you could add the new locations as 5-digit internal extensions, assuming of course that those numbers don't conflict with existing internal extensions (eg, if you current have extensions in the range 4600-4699, then you can't have 46500-46599 as a new range of internal extensions, unless you stop using 4650-4659).
If it's all too hard, you have to stop associating your in-dial ranges with internal extensions as matching numbers. There are downsides to doing this, so plan it properly.