Using a note that is not in the key

Is it possible to use a note that is not in the key. I am trying to use Hooktheory to learn my alto part in a song (Andrew Lloyd Webber medley) that is in Bflat and so far found that it works in the Lydian key (found this because I needed an E and not an Eflat.) Now I am need of a Csharp. Is this possible or should I just give up for this purpose.

Select a note and press . or , to move it chromatically.