You are describing the problem with inappropriate terminology. You are not "adding frequencies", you are combining signals.
Combining signals without loss can have different meanings and should be clarified.
1. No spectral loss, the signal must not be modified by the combiner
2. No power loss, the power of each modulator channel should be transferred to the common output without level attenuation
The second specification can be possibly achieved by passive filters, but the overall effort is most likely smaller if you use e.g. lossy directional couplers and an output amplifier. It should be considered that, depending on electrical length, 100 MHz frequency range probably demands for impedance matched design.