ANSWER: Assuming you qualify to open Solo 401k. That is, you are self-employed and have no full-time employees, yes you can rollover SEP IRA to Solo 401k then process an in-kind Roth Solo 401k conversion. Of course, you will need to pay taxes on the amount converted.
Further, you can make Solo 401k and Roth Solo 401k contributions; therefore, two Solo 401k bank accounts would need to opened, one for the ROTH and the other for the non-Roth.
For example, if the name of your Solo 401k is Osborn Solo 401k Trust, then the bank accounts would be titled as follows:
Bank Account 1. Osborn Solo 401k Trust
Bank Account 2. Osborn Solo 401k Trust (ROTH)
As you can see, it's the same plan, but two different bank accounts to track Roth vs. non-Roth.