Dear Sisters and Brothers in neuroscience

I am analyzing functional data in FSfast in Freesurfer. I define my conditions in an osgm file, then I create contrasts. On the second level, I would like to add two covariates (scanning facility and handedness). I don't need contrasts, I just need the model to account for the variance. Can I define both osgm and fsgd files? How do I construct the fsgd?

I will be grateful for your suggestions. 

Best,

Maria Czarnecka