I established a solo-401k in 2015 using Mark Nolan and mysolo401k.net. I have been a very satisfied customer.
My 401k has 3 accounts.
1. TD Ameritrade professionally managed brokerage account
2. TD Ameritrade self-managed brokerage account
3. Checking account at a local credit union
I have invested in Prosper with my 401k. As you probably know, Prosper is much like Lending Club. At the time (2015) Lending Club would not support a 401k account. My Prosper account was set up in the name of my 401k and was funded via ACH from TD Ameritrade (initiated from Prosper).
For me, there are no monthly fees and no ACH fees. One gotcha is that I cannot initiate ACH's from my 401k accounts. I can initiate outgoing wires and I can write checks.
Of late I have been moving funds out of Prosper via ACH to TD Ameritrade as loans are paid off. I was expecting double-digit returns from Prosper, and am very disappointed with my low single-digit results.
Prosper does issue an annual 1099 reporting earnings for my 401k, which I ignore for my tax filing.
I would suspect that you could deposit personal funds into your 401k's Lending Club account and call it a contribution, but I would check with Mark Nolan to be sure.
But it seems like to would be simpler and cleaner just to deposit the full $18K to your brokerage account and move funds into Lending Club from there.