Assuming x86/x64 here...
The stack pointer is tracked by ESP (32bit) or RSP (64bit) registers.
You can insert a mov ESP, ADDRESS
instruction into the ASM and change the stack to your preferred location. You'll need to store ESP/RSP on the new stack first because you need it for later.
Immediately after setting ESP/RSP call main()
(or any other C function for that matter) in the C program.
After function returns, restore ESP/RSP to it's original value.