The general rule to follow when modifying file definitions is not to change any fields in files that come with Order Entry. It is very difficult to predict which fields are changeable and which are not. There is no problem with adding more fields of your own to the end of the fields of a record.
There is usually no problem with adding additional files. This may cause problems in some screens because the screens are currently pushing system limits as to the number of files that can be open at one time. You just have to experiment in this area.