SQL & PL/SQL

I have made a data conversion from a LONG to a LONG RAW column. Every CHAR of the LONG column is converted to one byte in the LONG RAW. But for the aplication I need a conversion into two bytes
(e.g. 'T' shoulld be converted to '5400')

Now I tried UTL_RAW.CONVERT in the following way:

vraw := UTL_RAW.CONVERT(vraw,'UTF8','US7ASCII');

I get the following error:
ORA-06502: PL/SQL: numeric or value error
ORA-06512: at "SYS.UTL_RAW", line 261 in UTL_RAW line 261

In the next step I tried this:
vraw := UTL_RAW.CONVERT(vraw,'AMERICAN_AMERICA.UTF8','AMERICAN_AMERICA.US7ASCII');

I get no error, but:
The data are not converted from 1 byte to 2 bytes and I think I will need a West European Charset (or is there no difference to the American?).

Which dataset I have to use and it possible to make the conervsion in this way?

Thanks in advance :)
Carmen