It looks like you want to add a horizontal user-line. If the value of this line is known and static, then you can set it in the CR Designer by right-clicking on a y-axis label and bringing up the grid/scale dialog and then go to the last tab on the right. Here you can type in the value of a single user-line.
If you need dynamic user-lines, or multiple user-lines, you might want to look at our CRChart add-in product, which has superior support for this kind of stuff (see the @Y, @YSZ, @YSZN macros).
-Dan
disclaimer: I work for the company, threedgraphics.com, that makes CRChart. This product costs money.