What is the best way to model a cable (tension) joined in different pulleys in SAP2000?

Before considering this question you need to respect the fact that illustrated cables are flexible wire ropes (such as 7x19) which have very small elastic modulus (about 80 GPa to 140 GPa). So the cables might lead to no pronounced effect on the whole system. As a result, you might be able to simply neglect them.

Let's back to the question, modeling such system in SAP highly depends on the considered pulley details. As far as I got the details, following procedure might work.

Model the cables with SAP's cable element and model the pulleys (they are actually a couple of sheaves), with Hook link elements which is available in SAP. Hook link elements have two nodes. Connect the first node to the tower, and the second node to the intersection point of the cable elements. Note that Hook element acts exactly in contrast with the so called Gap elements. You need to use nonlinear analyses as cable and Hook elements are active only during nonlinear analyses. Set zero for initial Hook opening. 

This technique is approximate and its accuracy, as I said, highly depends upon details.

A good indication about accuracy of this technique is developed loads on the cable elements. They all should be the same considering a friction free sheave. Another verification tool is explicit modeling of the whole system in a general purpose FE software.