In RocPlane, there is no way to directly account for failure through rock bridges. What most people do is modify the cohesion to account for the rock bridges. See the following article:
Eberhardt, E., Stead, D., Coggan, J.S. (2004) "Numerical analysis of initiation and progressive failure in natural rock slopes - the 1991 Randa rockslide." International Journal of Rock Mechanics and Mining Sciences (41)69-87.
If you use Mohr-Coulomb, then you can easily increase the strength through the cohesion and friction terms as shown in Figure 2 of the above paper. If you are using Barton Bandis, what you may need to do is fit a Mohr-Coulomb envelope to the Barton-Bandis envelope. This can be done using RocData. The cohesion and friction would then need to be modified to account for the rock bridges.
Another option is to use Phase2, See verification #17, #18, and #19 in the Phase2 Joint Verification Manual.