Catch-up contributions can only be made to plans with salary deferral features.
If you want to make catch-up contributions, you will need to amend your profit-sharing plan to include a salary deferral feature if it does not already have one. Most profit-sharing documentation includes a check-box option for adding a salary deferral/401(k) feature to the plan. Check yours to see whether you have already made that election. If you did not, the deadline for making this change will be December 31 each year for calendar year plans.
For more insight, read Retirement Savings Tips For 45- To 54-Year-Olds.
This question was answered by Denise Appleby