Yes, it is.

You'll just have to define when they occured.

I was said that it was better to use raw data as covariates at the 1st lvl design than to use mean RT at the 2nd lvl.

what's the best way ?

Depends a bit on what you want to do and know. If you want to control for the effect of covariates on single subject data, then it's better to specify these in a 1st lvl design. If you want to know the effects of covariates in a group analysis, you would specify those in a 2nd lvl design. However, if there are large differences in RT between subjects, you may wonder, whether the mean RT says anything useful at all.

Cheers,

Wouter