It sounds like you are trying to export records from your old system INTO CRM so I don't understand why you are asking about exporting FROM CRM?
But anyway it sounds like you are exporting from CRM via an Advanced Find - Click the Excel button once you have run the query and you will be able to select 'Static worksheet with records from all pages in the current view'
This will export all the records rather than the first 250.
If this isn't what you mean please let me know.