Including covariate variable in script

Hi everyone!
I have a question about how I include a covariate variable in my analysis.
I have three categories with datapoints from each participant, with each timepoint specified (each timepoint represents activity during a word).

I would like to include the frequency of words as a regressor/covariate variable in the analysis, but I am not sure how to include this as a file/ and in my script. If I included a new regressor, this would be applied to all three categories, but the specific frequencies of the words apply to the specific words in the three categories.

Thank you so much in advance!

Do you have a starting command that you are working with already? It will likely be important to know such details to be able to answer more effectively.