If you cannot or if you do not want to use macros, you might want to interpolate with these worksheet functions (poor you):
Poor man's interpolation (PERCENTILE / PERCENTRANK approach)
Poorest man's interpolation (piecewise TREND approach)
If you are lucky and your slopes between adjacent points are monotonously increasing or decreasing you can apply the funnily short MiniMax Interpolation.
If you lie to fill gaps with linear interpolation using worksheet functions, look here.
If you are interested in downloading a 28 KB Excel 2010 sample file which was used to create the examples on this page go to my Download page, please.