How to modelate foundation beam in SAP2000?

you can take reactions at supports which are multiply by (-1) and apply them at there position on the beam.the value of spring stiffness which modelise soil must be multiply by the width of fondation (beam). the precision of calcul depend on the number of division of beam.
anothere way is to place the grid of beam at the bottom of structure and at the place of support you place spring. As above the precision depend on the number of spring you place to modelise soil.
hope this help.

As DENSAL suggested, why not assign springs to your beams and/or basemat foundation based on subgrade modulus of the soil? SAP lets you make line/lineal spring assignments to frames and area spring assignments to shells, and the program automatically keeps track of this assignment no matter if you change the mesh later.