Keep in mind that the IRS has an annual additions limit, I believe it is 51000 in 2014. All of your 401k and employer profit sharing contributions will count toward this limit, whether they are in your workplace plan or in your Solok/ SEP IRA. Also, you're 401k deferrals are still limited to 17,500 in 2014 (plus catch up if you are over 50), this will include all plans you have access to. My point is, if you already have a 401k through work, adding on the side may not increase your ability to defer money. If anything, it may allow you to make an employer profit sharing contribution for yourself from your side business
As for the source of the money, only the IRA allows you to defer money from any source. The 401k and other retirement plans require you to use payroll or profits from the company that sponsors the plan.