If you are using the "rugarch" package in R, you can include these terms via the argument external.regressors within the argument mean.model in the ugarchspec function.
From CRAN:
external.regressors A matrix object containing the external regressors to
include in the mean equation with as many rows as will be included in the data
(which is passed in the fit function).
For further reference: https://otexts.com/fpp2/seasonal-arima.html
