Level1Design: don't pass onsets, but only regressors

I want to create my own design matrix with onsets having different amplitudes (instead of just a vector of ones). This you can’t do in SPM, at least I don’t know how to. So in nipype I just set the the conditions, names and durations to [] or None in the Bunch object, and instead inserted the preconvolved design matrix as extra regressors. The problem is that these aren’t interable and that’s exactly the error I get. Is there a way to get around this or do I have to edit the code?