Use a table. Example:
Code:
table <boss.block> file "/path/to/a/file/containing/addresses/and/or/cidrs"
block from <boss.block>
block to <boss.block>
For more provisioning information, see the Tables section of the PF User's Guide, and for complete information, the pf.conf(5) man page.
Edited to add: tables can also be managed with pfctl(8). You can add/delete, reload an edited file, replace with a new file, etc. See the pfctl(8) man page and the -t/-T options.