Basically, when my system is running, I would like the user to ftp some new code to the SD card, and dynamically load the new function and create a task to run in the system. This is normal for Linux. For example, I can compile a SO, and dynamically load into the memory.
How to do it in uC/OS II or III?
This is not a service uC/OS-II or uC/OS-III can provide by itself.
You would need a program loader that is able to read an ELF file, copy its relevant sections (ex .text, .rodata, etc.) in memory according to load addresses specified and allocate memory for uninitialized memory sections. You would then be able to create a new uC/OS task and pass it the function pointer that corresponds to the ELF entry point.
Most embedded systems don't have a Memory Management Unit (MMU) and thus you would need to pay special care to the linking process to make sure any of those sections don't overlap with any code that would already running on your target. Depending on your toolchain, that would most likely involve carefully crafting your linker script.
Another option that would avoid the problem of potential overlapping of the memory space would be to use a toolchain that can produce position independent code and load the ELF in the heap of your main application or in any other allocated and available memory space that is allocated by your main application.
Yet another option would be to produce relocatable code and use or build a program linker that is able to process relocations at runtime, when you want to load the uploaded code.