To the best of my recollection your single primary - five
secondary transformer can be modeled in SPICE. But, it's a
bear, involving writing terms for all the mutual coupling
cefficients. I expect you would get 5-factorial terms (120).
Cadence's Spectre behavioral language and AWB product would
be a bit more tractable. But, I would also look at Analogy's
SABER/MAST product. They might even have a primitive set up.