You're looking for something along these lines...
Code:
#STMT 1
nat on $ext_if inet tagged MYIN2OUT -> ($ext_if:0)
#
# STMT 2
pass in log quick on $int_if inet \
from ($int_if:network) to any \
tag MYIN2OUT \
keep state
#
# STMT 3
pass out log quick on $ext_if inet \
tagged MYIN2OUT \
keep state
#
In the order of precedence, your traffic originating on your $int_if:network and destined for the internet is
FIRST inspected and passed by SMTM#2, then NAT translation occurs by STMT#1, and is then inspected and passed by STMT#3.
The use of
tag/
tagged means you -- the pf.conf author -- don't need to know the real IP source/dest translations at the point of each statement to write a clean (and leak-proof) pf.conf.
/S