A .dmp file from Postgres is a regular plain text file so you should be able to use e.g. iconv. Say you wanted to switch from UTF-8 to LATIN1, you could do:
iconv -f utf-8 -t latin1 <infile >outfile
Please note though that this will only change the encoding of the characters. If you need to change the DDL that is used to create the tables, columns etc, you will need to modify the file manually. Or import it, make the changes and then export again.