The SESSION encoding (what you see in the ENCODING system option) has to be set when SAS starts.
So to change that you would need to get your SAS admin to create a application server using a different session encoding and connect to that application server with SAS/STudio instead of the one you are using.
If you have to work with multiple text sources you are probably going to be better off just moving directly to Unicode support. Encoding=UTF-8 instead. Any single byte encoding is going to be limited to 256 different characters.
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