With more research here is what I found,
1. 401K Makeup contribution is allowed only for Veterans under USERRA Act.
2. For those type of makeup contributions for previous years, IRS does provide the option to enter the 2 digit year, to indicate 401K employee contribution for past years (ex: D 17 D 16) etc. This information is also stated in IRS document GeneralInstructionsForW2 ()iw2w3) Page 18
3. Even for these type of makeup contributions, IRS does not say how those have to be reflected in the tax form. Past years tax return has to be amended for sure.
3. Since I am not a veteran, my employer obviously has made a mistake. Have reached out to them with these addl. info. Will update on how it goes.
