It is correct but doesn't say what you want it to say. The problem is that it can be interpreted as saying that she is already hopeful - she already has hope so she is not seeking help. To express the idea that she is looking for help, say "She smiled at him hoping for help".

It's a little clumsy, but it's understandable. The first example is the most concise: "She smiled at him as if to ask for help." "She smiled at him as if appealing for help" "As if to appeal for help, she smiled him." "Like a subtle plea for help, she smiled at him."