You can make use of the standard tables in BW TCURR,
Maintain the values in TCURR.
Create your currency translation type in RSCURR tcode and maintain the same in the TCURR table.
Give this translation type in the Query level and get the values converted.
Please let me know if there is any reason for maintaining the values in the Ztable. If yes we need to go with customer exits.