you would have to make a mod that replaces the ANIM.pbo, but then all the units would walk like that...
OR
Define new weapons to use custom anims, but then the units would only walk like that with those weapons...
that only works for normal walking, otherwise it cant be done (we cant ADD new walking type animations, we can add cargo or switchmove anims, but not walking. Also we cannot map new animations to keys (like k for lean left and l for lean right)
Making units act like SWAT teams, or Delta teams is not really possible... as that would require altering the game engine to make the soliders act more stealthy in a CQB environment