The basic compilation model of GNAT requires that a file submitted to the compiler have only one unit and there be a strict correspondence between the file name and the unit name.
The gnatchop utility allows both of these rules to be relaxed, allowing GNAT to process files which contain multiple compilation units and files with arbitrary file names. gnatchop reads the specified file and generates one or more output files, containing one unit per file. The unit and the file name correspond, as required by GNAT.
If you want to permanently restructure a set of 'foreign' files so that they match the GNAT rules, and do the remaining development using the GNAT structure, you can simply use `gnatchop' once, generate the new set of files and work with them from that point on.
Alternatively, if you want to keep your files in the 'foreign' format, perhaps to maintain compatibility with some other Ada compilation system, you can set up a procedure where you use `gnatchop' each time you compile, regarding the source files that it writes as temporary files that you throw away.
Note that if your file containing multiple units starts with a byte order mark (BOM) specifying UTF-8 encoding, then the files generated by gnatchop will each start with a copy of this BOM, meaning that they can be compiled automatically in UTF-8 mode without needing to specify an explicit encoding.