Okay if you have one per session than it is not as easy as that. 
The problem is that the array containing the covariate needs to have the same length as your number of subjects.
So if you have an array 'All' with 10 subjects, you cannot have a covariate 'FDprepost' containing 20 values. In that case you need to create two separate covariates. Alternativel  you could also think again about using the FD measures as covariates in the first level as Alfonso suggested but that is just my intuitive idea (note that I myself use the toolbox for not such a very long time).

Best,
Julia