In your code you did not assign output of CE_CONVERSION to any temp table.
Here is a code snippet that you can use in your method body.
PRICES = select 'AA' as id, '100' as in_amount from dummy;
lt_uom_std_conv = CE_CONVERSION (
:PRICES,
[ family = 'currency',
method = 'ERP',
steps = 'shift,convert,shift_back',
target_unit = 'EUR',
client = '800',
source_unit = 'USD',
reference_date = '01.07.2014',
error_handling = 'keep unconverted' ],
[ in_amount AS out_amount ] );
et_amount = select out_amount as BAPICURR from :lt_uom_std_conv;