You can fit 205 tyres onto a 215 rim, however, I must stress it is not recommended. You are not advised to take any corners at great speed and as a few guys I know in the tyre fitting trade say that fitting anything other than the proper size tyres to your rims will mean any stress placed on the tyre forcing the tyre to come away from the rim wall and blowing out.
Stick to the proper tyre size is my recommendation