What you can do is have a sketch with all the points that you need to create the spline, then exit the sketch and utilize those points with a Studio Spline.
To use an expression you would need an expression for at least each of the x and y points, assuming the points all lie in the same Z plane.
Otherwise you could have an expression used for the plane distance from the x-y plane and within each sketch the dimensions for each direction would reference the relevant expression.
I don't think you can set point coordinates as an expression, at least not with UG 7.5
In NX 7.5 you can either pre-create a series of Point 'features' (Insert -> Datum/Point -> Point...) with the 'Associative' option toggled ON and then create a Studio Spline thru Points selecting these Point objects.
Or you can go directly to the Studio Spline thru Points function and when you're asked to 'Specify point location...' select the last icon in the Snap Point section of the Selection Bar labeled 'Point Dialog' and when the dialog comes up change the 'Type' to '= By Expression' where you will now be able to directly create 'Point Expressions' for each one of the Spline Points, resulting in a fully 'parametric' Spline.
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