x1 = [2 3 5];
y1 = [1 2 3];
y2 = [1.5 2.5];
x2 = interp1(y1,x1,y2)
Here is a simple example using the interp1 function. I'm not certain if this is what you mean.
If you look at that documentation page, note that the use of "x" and "y" variables are swapped compared to what is written here, because the normal convention is that y is dependent on x, but that is not what you have described in your question.
If this doesn't do what you want, or if it is unclear, then I suggest you actually load your data from Excel into MATLAB, save to a *.mat file, and upload those data here.