If you just want a bytea value corresponding to the string's byte sequence, you can use:
SELECT convert_to('åbçd€','SQL_ASCII')
If this is isn't quite what you're looking for, you can convert to a set of codepoints, and from there, you can do what you want with it:
SELECT ascii(c) FROM regexp_split_to_table('åbçd€','') s(c)
Note that these are very different in the way that they handle non-ASCII characters. Assuming a UTF8-encoded database, convert_to('å','SQL_ASCII') will give you multiple UTF8 code units, while ascii('å') returns a single Unicode codepoint.