Insert Variables In AT commandes
I cannot find which "SIM900.h" library you are using - none of the ones I found have "SimpleWriteln" in them, so this is pure assumption about what functions and overloads may be available.

Just split your string into chunks, like you would with any serial device:

gsm.SimpleWrite("AT+CUSD=1,\"");
gsm.SimpleWrite(command);
gsm.SimpleWriteln("\"");


Where command is a suitable container for your data - say const char *.