Is it possible for you to provide a feature where you can provide an excel template and users would fill in the information taken from other non mt4 broker sites. This excel file can then be uploaded by the user so that it can populate the fields in your database.
This feature helps so many users who are on non mt4 platforms until you were able to provide systematic access to them.
We can set up an account, download the template (probably mimics the same fields that are being uploaded by the MT4 Statement.html), populate the template with formatted data downloaded from their brokers, click on a button that would say Upload and provide the location of the file. It is the user responsibility to make sure he is not uploading any of the existing transactions to make sure there are no duplicates.