For most of your work with boundaries, STAR-CCM+ opens a dialog like this one. It allows selection of multiple boundaries from various regions.
Regions list box - a list of all the regions of the mesh.
Boundaries list box - a list of all the boundaries for the region that you select in the Regions list box.
Selected list box - the boundaries that have been selected.
>> - Transfers all items in the Boundaries list box from there to the Selected list box.
> - Transfers the item that you select in the Boundaries list box from there to the Selected list box.
< - Transfers the item that you select in the Selected list box from there to the Boundaries list box.
<< - Transfers all items in the Selected list box from there to the Boundaries list box.
Apply - Splits the boundaries that you moved to the Selected list box.