Yes, it is possible that you can contribute to a traditional and/or Roth IRA even if you participate in an employer-sponsored retirement plan (including a SEP or SIMPLE IRA plan). 

To make a Roth contribution, the taxpayer must be below the MAGI phaseout level. A taxpayer can always make a nondeductible traditional IRA contribution when covered by a retirement plan at work. Depending on the taxpayer's income level, the deduction may be fully, partially or entirely nondeductible. These limitations change each year, but can be found in the Form 1040 Instructions.