Bob writes:

<<As TMFPixy writes, it is possible, but I believe that the funds may then be considered "commingled" so that you will lose the ability to later transfer them to a qualified retirement plan,>>

No, that's not true. If the source of the contributions is from an employer-provided qualified retirement plan from one or more employers, then co-mingling does not occur. While you cannot mix 403b plan money with 401k, you can mix one or more deposits coming from multiple 401k plans and/or defined benefit plans. All of that money retains its eligibility for a subsequent transfer to a new employer's plan that accepts such transfers.

Regards..Pixy