Below I linked a discussion from over on the Iris forum on how to grow them from seed. Siberian work just the same as flag iris. Stratification is required and then start them indoors. Good luck.

Dave

Here is a link that might be useful: growing iris from seed

I winter sowed Siberian iris seeds I harvested from my own plants and got good germination. They grew to about 12" tall the first year and grew to their full height in year two. I had a single flower on the plants grown from seed their second year but expect to see several blooms this year.

According to the Clothier database, they do need cold stratification in order to germinate which makes them excellent winter sowing candidates. Check out the Winter Sowing forum here on GardenWeb for any questions on how to go about it. It's very simple and inexpensive.