You cannot exceed the $15k that she earned through the job. But you're not actually going to be able to stick the entire $15k in there either.
She still has to pay the ~15% tax for both sides of Social Security (and the related taxes). You can reduce how much of her earnings is taxable for this by having her first maximize the employer side of the contributions, which allow for 15-25% of the income to be contributed and deducted as a business expense before calculating how much she needs to pay for the SSN+related taxes. Exactly how much she can contribute varies a bit based on a lot of factors, so I'd recommend looking around to find out how to do the math, or taking it to your accountant so they can figure it out for you.
Assuming the total is $15k, then she'll pay in x% as a business expense/contribution, be subject to ~15% on the remaining (100-x)%, and be able to contribute as an employee for the amount left after both those taxes and expenses/contributions are taken out.
Ex: If your math/accountant revealed that the legal limit sets x at 20%. Then she'll do $3k as an employer contribution, pay 15% of $12k to social taxes ($1800), and then make a employee contribution of up to $10,200 (which, by her contributing it, is not subject to other income taxes).
Confusing enough for you?
Again, the answer to your first question is: No, you cannot use extra money from other sources to increase the contribution amount.
