If the WAERK (document currency) is same as WAERS, then as already suggested, you can input the field KNUMV in VBAK where you get WAERK and get KNUMV value. Pass this KNUMV to KONV so that you will get the desired result.
On the other hand, if WAERK would be different in some cases, then you can straightaway take the WAERS from program RMCV0100 where all Info Structure data would be stored.
G. Lakshmipathi