Yes. As long as your plan permits, you can rollover other retirement plans to a Solo 401k. This allows you the ability to consolidate retirement accounts.
Retirement accounts that are permitted to be rolled over into a Solo 401k include a 401k, 403b, 457 and Thrift Savings Plan from a prior employer. You can also rollover a SEP IRA, SIMPLE IRA, Keogh plans, Defined Benefit Plans, Traditional IRA and Rollover IRA.
A Roth 401k from a prior employer may be eligible to be rolled over into a Solo Roth 401k provided the 401k plan document permits Roth 401k contributions and rollovers. However, IRS rules do not allow a Roth IRA to be rolled over into a Solo Roth 401k.