RSIMPCURR to run for multiple R/3 systems

We have one BW system being fed by 3 different ECC systems. Each ECC system has it's own table of currency exchanges. One system converting to Euros, a second system converting to British Pounds and a third system converting to US Dollars. We need BW to be updated with data from all three tables.

From what I can glean from documentation, the variant for source system for the RSIMPCURR program cannot be set in the ABAP variant screen but can only be set from the Source System tree in the BW workbench - at which point you select one and only one source. Is there anyway to modify the process to allow BW to be updated from multiple source systems?