Yes, you can feed your signal through an attenuator to reduce it to the level that's safe for the SDR. The amount of attenuation depends on the maximum level specified for the SDR. You can use the formula, dB = 20 log(Vin / Vout), to calculate the needed attenuation. Alternatively you can feed the signal into a resistor and then place the SDR antenna in the vicinity of the resistor to pick up the radiated signal. This approach is less consistent or repeatable, but I use it frequently to look at spectral shapes.