The best way to do this, since you have already exported the mesh in Salome is to generate groups of faces in your Salome mesh. Since you have no geometry to base your choices on (e.g. group all the faces that belong to a circle A), you will have to use filters.
Go to Mesh->Create Group->Group on Filter, and set such filters that will enable you to isolate the faces of interest. For boundary faces, start with the filter Free faces. You may create additional geometry, such as planes, or rectangles, that you know will determine your set of faces.
Even though all you have is a Mesh in Salome, you can create geometrical entities that you will then use in the Set_filter environment to filter out the faces.
Example of filters:
Free faces + Belong to Geom
Where Geom is e.g. a rectangle that you have drawn in the Geom module. This is the simplest and most elegant way, you just need to know the dimensions of your geometry (which you must know, if you are running a simulation on it). Once you have created the Geoms, they will be selectable: just click on the Threshold value of the Belong to Geom filter and select the geometrical entity from the Geometry tree on the left.