I presume you want your spaceship to rotate Y (from your expression) whilst using a curve as a motion path?

You will need to first apply the motion path, which will turn your translates and rotates in the channel box to yellow to show they are effected. Right click on the rotateY in the channel box and select 'break connections', this will turn it back to white, now you can apply your expression to rotateY. Whilst still having the other rotates be affected by the motion path so you can use banking and such.

Chris