Is it possible? I know it depends on legal issues; it is just for testing.
I will "transmit" from RPI (Raspberry pi as fm transmitter).
Amateur Radio Stack Exchange is a question and answer site for amateur radio enthusiasts. It only takes a minute to sign up.Sign up to join this community
An audio amplifier is not designed to have the bandwidth to amplify RF — it will not pass or amplify signals much greater than audible frequencies, which would require significantly different design. In fact, if it did amplify RF, it would be a potential source of RF interference by amplifying incidental noise on the input.
If you did find an amplifier to use, then you should also use a band-pass filter for the desired output frequency, placed between the amplifier and the antenna. This is always a good idea, but especially necessary in this case because the Pi being used in this way produces a square wave, which by definition has harmonics — signals at multiples of the original frequency; e.g. starting from 100 MHz, also 300 MHz, 500 MHz, 700 MHz. If you amplify those too, you interfere with frequency bands unrelated to the one you wanted.
However, it is likely that adding any amplification at all would be illegal even if your original transmission isn't, because unlicensed transmissions are limited to very low power. Here's a question about the power limits in the US, for example.