THnSparse fit

Dear Rooters,
I’ve built a 6D THnSparse via:

Int_t bins[6] = {3,3,3,3,3,3}; Double_t xmin[6] = {0,0,0,0,0,0}; Double_t xmax[6] = {3,3,3,3,3,3}; THnSparseD hs("hs", "hs", 6, bins, xmin, xmax);

and filled it through

Double_t x[6] ; hs.Fill(x, weight);

I need to fit the THnSparse with a 6d polynomial function.
How can I define such a function since no TF6 is available?

Thank you,