I'm working on an SDR project where I need to transmit a narrow
baseband signal on multiple different carriers simultaneously, over a
relatively wide bandwidth (1Mhz). … an external audio
source is then AM modulated and transmitted on several channels …
In my case the output channels are arbitrary, …
If you do not need precisely selected frequencies and amplitudes, a computationally cheap way to generate many copies of a signal is to multiply the baseband by a carrier signal high in harmonics (such as a square or sawtooth wave). The signal will appear at the frequency (and amplitude) of each harmonic. In order for the spectrum to be otherwise clean, make sure the carrier frequency is an even division of the sample rate.
This is similar to the effect where a signal overloading a SDR receiver will appear duplicated across the received spectrum.
harmonic-signal-duplicator.grc