Hello,
The FM "EPS_OPEN_OUTPUT_FILE" seems to be a RFC. I would suggest to use DATASET commands for writing to the app server.
Check this link
http://help.sap.com/saphelp_nw04/helpdata/en/fc/eb3ca6358411d1829f0000e829fbfe/frameset.htmhttp://help.sap.com/saphelp_nw04/helpdata/en/fc/eb3ca6358411d1829f0000e829fbfe/frameset.htm
Best regards,
swanand