Set to run a program within another program using NXT-G
As far as I remember there is no possibility within NXT-G to programmatically execute another program. Moreover, as I understand it the default firmware simply doesn't have that possibility (the command to start a program is a direct command which must be sent via bluetooth and can't be used in a program). Maybe other programming languages using other firmware can do it, but I don't know about that.

However, you could encapsulate certain parts of your program in your own blocks, which I would say are similar to C functions. As such, it will simplify your main program, and you can reuse blocks in different programs (but the blocks themselves are included in the program file, it's not like a library which would be a different file). You should probably read a tutorial on how to make your own blocks to decide if it's a valid solution for you.