Right ...
Gang nail trusses are designed to take compressive and tensile loads in the members. The strength of the joints (gang nail plates) is determined by trial and error rather than calculation, thus it's a bit of a black art.
Bending stresses in individual members are induced by UDLs (uniformly distributed loads) which can be dead or live. The UDL in the top chord of the truss is composed of the dead weight of the roof tiles, battens etc. and the live load of snow and wind. The bottom chord of the truss is designed to take the dead load of ceiling finishes, mechanical/electrical services etc. and the live load of access for maintenance and repair. The raking members of the truss are simply in compression or tension with no induced bending stresses.
What you are proposing will induce increased bending stress in the bottom chord of the truss whilst increasing the overall loading on the system and increase the tensile and compressive loads in all the members. This could be a bad thing as sooner or later you will overload the system.
However, factors of safety are built in, 1.4 for dead loads and 1.6 for live loads IIRC, so providing you don't increase the load too much you should be OK
HTH