First of all I don't have the Rigol 815. But I'm pretty sure it works down to it's specified frequency whatever that is.
The question is why do you want to use such a low frequency and are you sure that you really need a SA?
If you don't need the high dynamic range than you can use an oscilloscope + FFT.
If you do need the high dynamic range than are you sure that you want 50 ohm input impedance? Do you have an active probe for example?
Are you really sure that you want to use this as a low frequency RF network analyzer for 50 ohm characteristic impedance signals? Because that's a pretty uncommon application.
If you need high impedance inputs and the high dynamic range then consider dynamic signal analyzers or audio analyzers.
You can find them on Ebay easily. You might even consider something like this:
https://www.quantasylum.com/content/Products/QA400.aspx