man postfix-policyd-spf-perl will tell you how to set this up. It certainly doesn't qualify as easy, but, at least for a list of IP addresses it can be done. See the paragraph that starts "The policy server skips ..." in the synopsis.
I agree it's not as easy or functional as it should be. If you look at postfix-policyd-spf-python (be sure to get version 0.6 - currently in unstable) it has the features you are looking for in a config file installed in /etc. Putting my upstream hat on (I'm also upstream for both these packages), patches welcome for the Perl package. My Perl is very weak and so the development is being done on the Python package except where people are willing to contribute.
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