You can roll over multiple old and “orphan” retirement plans (e.g. Rollover IRA, SEP-IRA, SIMPLE IRA, Traditional IRA, etc.) into your Solo 401(k). You also have the option to roll over funds in part or full in your old accounts into your Solo 401(k). And, yes, you maintain the same tax-deferred status within the Solo 401(k). The only account type that may not be rolled over into a Solo 401(k) is a Roth IRA.
Find the IRS Rollover Chart here.