1. It could be permissible, if it may be assumed that, (a) the additional labor categories are determined by the KO to be within the scope of the original contract/TO, (b) adding them meets a bona fide need, (c) there was (when the TO was awarded) and still is adequate funding available, (d) the additional labor categories being added are in the best interest of the government, and (e) the parties can negotiate a fair & reasonable bi-lateral change to the contract.
2. It would not be permissible however, (a) if adding the categories were not within scope, or if a new justification and authorization (J&A) could not be obtained to add them, (b) they don’t meet the Bona Fide Need Rule, (c) adequate funding was not available when the TO was awarded or isn’t available now, (d) they’re not in the best interest of the government, or, (e) the parties are unable to negotiate fair and reasonable labor rates (if there isn’t a Forward Pricing Rate Agreement (FPRA) in place.

However, if contractors are routinely making such requests, it could be that the labor categories that they’re using in their proposals are purposely set low in an effort to win the task order. They then may attempt to get well once the award is made by using additional labor categories. So if your technical personnel are substantiating the original labor categories negotiated as sufficient to meet the requirements of the LOE TO, then it might be more prudent to not allow the winning contractor to add additional labor categories or to add additional personnel into existing labor categories unless the FTEs originally negotiated included them.