Remco,
you can use the oracle function convert
select ....., convert(column-name,'us7ascii','utf8') as ......,
from ....
CONVERT(string_expression,to_data_set[,from_data_set]);
you can read from the docs
https://docs.oracle.com/database/121/SQLRF/functions041.htm#SQLRF00620
Common character sets
Character SetDescription
AL32UTF8Unicode 5.0 Universal character set UTF-8 encoding form
EE8MSWIN1250Microsoft Windows East European Code Page 1250
JA16SJISTILDEJapanese Shift-JIS Character Set, compatible with MS Code Page 932
US7ASCIIUS 7-bit ASCII character set
UTF8Unicode 3.0 Universal character set CESU-8 encoding form
WE8EBCDIC1047IBM West European EBCDIC Code Page 1047
WE8ISO8859P1ISO 8859-1 West European 8-bit character set
WE8MSWIN1252Microsoft Windows West European Code Page 1252
ZHT16MSWIN950Microsoft Windows Traditional Chinese Code Page 950
Hope it helps you
Best regards
Carl Ruhle