You can use the preforms and freeze yeast. I have 20 ml vials so I use 5 ml yeast, 5 ml glycerin and 10 ml water. Mix them well, chill in the refrigerator over night, mix again and freeze. With the bigger vials you can make larger samples. I then thaw them and make a 250 ml starter of 1.020 wort, for step one then a 500 ml step then a final step to get the size for the recipe. I takes about a week.
I have read that 4 to 10 generations is the max. I make 4 vials and if each time I used one I made up 4 more for 4 generations, I could brew 256 times from the original purchase.
I have successfully used yeast stored for over a year.