Fixed effects (FFX) analysis over subs using SpecifySparseModel

I’ve been thinking about how to create a fixed effects analysis over subjects (I’m only interested in the average group effect and not the population effect) in nipype. Since data are in a sparse design format I would need to use SpecifySparseModel in the pipeline. The model specification creates one design for each subject. How can I make it create one design over all subjects and runs?

So far I thougth about merging all runs over subjects once they come out of the normalization step in the pipeline. I’m not sure if this is possible in the nipype interface. I suppose I would need to trick SpecifySparseModel to think that they are just different runs for just one subject.

I would greatly appreciate some pointers here. Thanks!