Assuming that you want your mapping to execute one of those buffer commands, use this:
:map \ff :badd
Make sure that you include a space character after ":badd". See
:help 05.3
:help map.txt
If you meant that your mapping should execute both of those commands, then use this:
:command -nargs=1 BuffAdd badd <args> <bar> b <args>
:map \ff :BufFAdd
Again, include a space after ":BufAdd" in the mapping. See
:help 40.2
:help user-commands
For more complicated tasks or argument-handling, you could write a function. See
:help 41.7
:help user-functions
A function that included the use of the input() function could allow you to type \ff
followed by the buffer name without seeing :BufAdd
on the command line, like this:
function MyFunc()
let my_buffer_name = input("Buffer name: ")
exe 'badd' my_buffer_name
exe 'b' my_buffer_name
endfunction
map \ff :call MyFunc()<CR>