There is an easy method to create disulfide bonds in a protein.
In the Macromolecules Tools panel: Design Protein > Predict and Create Disulfide Bridges.
This tool finds pairs of residues in the structure, which if mutated to cysteine, could form disulfide bonds with reasonable geometry. The predicted sites are listed in the Datatable along with calculated properties which will help with assessing whether the mutation is likely to stabilize the structure.

In the Tool panel, click the up/down arrows to step through the predicted sites and create the disulfide bridges.
For more details of the method, review the theory section in the DS Help: Macromolecules tools > Theory - Macromolecules > Theory - Predict Disulfide Bridges