Dear Alfonso, 

I had exactly the same needs so I implemented what you suggested in this post. 
This generated an SPM.mat file, together with a spmF_mv.nii, spmF_mv.json and mask.nii files. 
I am now trying to load the results with conn_display SPM.mat, however I get the following error: 

Loading /rds/user/sb2506/hpc-work/tFMRI_PPI/2ndlevel/dep/SPM.mat
please wait...
Error using regexprep
All cells must be char row vectors.

Error in conn_displayroi (line 204)
data.names=regexprep(data.names,{'_1_1$','^rs\.','^rsREL\.','^aal\.'},'');
Error in conn_display (line 209)
hfigure=conn_displayroi('initspm',fullfile(filepath,[filename,fileext]),varargin{:});

Any idea where this might come from?