There are really tons of solutions for this problem, And while I see why they ban the hotmail and yahoo addresses (I would probably do so myself if I ran a forum), they really should tell you that when the account is rejected.
Anyway, You have several options, but I will only list the easyest.
Forwarding, there are tons of places you can get an address and have it forwarded to your existing email address.
One place that comes to mind is cjb.net. You can go there, set up an account for somename.cjb.net, and have it forward to your hotmail address.
You can then use firstname.lastname@example.org
, and the email will be forwarded to hotmail.