I'm not certain that I understand your question exactly. most of the monitor programs will let you download code to ram via a serial port. so getting the code to the RAM should not be an issue.
However, if you are asking how you can then execute it. You can't if you have implemented a pure harvard architecture design. That is, you can rewd and write to data RAM but you can't execute any instructions from it. in Code memory you can read it and you can execute it, but the processor can't directly write to it.
The solution in this case is to go back to a Von Neuman architecture that is you no longer maintain separate code and data space. then you can read, write and execute from the memory.
You don't necessarily need to do this to the entire design. I designed an 8052 micro board which maintained a harvard architecture for the lower 32K of memory but used a von neuman architecture for the upper 32K of addressing. In this fashion you could now address 32K of Code only space, 32K of Data only space and 32K which could be either code or data.
I hope this helps, ------ SAKER