googling internet about using certificate and keys from PKCS12 file lead to convert it to PEM.
it requires some additional utilities (openssl not default windows application) and manual steps to convert.

this patch allow using PKCS12 directly from stunnel.
example configuration:
cert = /home/dab/.certs/my.p12

PKCS12-files detected by file extension (.p12 or .pfx, no case sensetive) and load specially.
password prompted if needed, empty password also supported.

code based on examples from:
https://groups.google.com/forum/#!topic/mailing.openssl.users/iuBmSqwsIG4
http://openssl-users.openssl.narkive.com/J0bR3cMA/ssl-ctx-use-privatekey-file

patch tested on stunnel working on linux (gentoo: from our overlay http://bbgentoo.ilb.ru/repos/bbgentoo/bbgentoo_overlay/branches/drafts/net-misc/stunnel/)
and windows.