First you need to convert every .hgt file into the .tdb format. Then there is the following way to combine multiple topo files in ProMan which can then be used for simulations:
Load the first *.tdb file in ProMan and combine the second *.tdb file via Edit => Combine Data => Max Value
Fill the undefined pixels using Edit => Modify Data in Area => Interpolate Data
Save the resulting topo data into a new *.tdb file via Data => Topo Database => Save As
For rural scenarios the predictions can be also done for individual Rx points (under the simulation tab you can enter the points). If the maps are in metrical UTM coordinates you need to convert first long/lat to UTM (in ProMan under Data => Convert Coordinate Datum).