Retirement Account funded by Cash Flow

[Steven Molczyk]

I have a few properties, definitely not considered active/earned income at this time, but is there any way to funnel my cash flow into a Roth investment vehicle (other than Roth IRA, I'm already funding a backdoor Roth separately)? I know that I can't use a solo-401k until it becomes earned income but I don't plan on quitting my W2 for the foreseeable future so that's not an option.

Thanks for any ideas!