Can my wife transfer her 401k from a former employer to my Solo 401k plan?

Answer:

No, she cannot rollover her retirement funds into your account. All retirement accounts are individual and tied to a Social Security Number, so she would not be able to rollover her IRA or 401k into your individual account. (The only exception would be upon death).

However, your wife can participate in your Solo 401k if she is also employed by the business. If she participates in the Solo 401k, she can have her own participant account.  She can then rollover her IRA or 401k into her own account under the same Solo 401k plan.