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The SEP is actually a traditional IRA that has the added function of allowing employer contributions.
You can continue to make contributions to the SEP only if you have income from self employment as a contractor. See http://www.irs.gov/retirement/participant/article/0,,id=211336,00.html
Or, you can combine the SEP into a traditional IRA and make contributions the same as any traditional IRA. Use a trustee to trustee transfer to avoid withholding.
Please ask if you need clarification.