Steve,
Unfortunately this will increase the rate of oxidation as the powder will have more surface area than the pellets, giving oxygen more contact with the tren. I think if you mixed the tren with the transdermal formula, it might act to preserve the tren somewhat.
The expiration on the fina is also probably quite conservative, the army up until recently was throwing away millions of dollars of "expired" drugs every year. They had some old, "expired" drugs tested well after the expiration date and found there to be little to no loss in potency. I suspect this would apply to the finaplix also, so if you go 6 months over the expiration, or even a year I doubt you will lose enough to worry about.
Link Removed