on a quick glance, i see 2 possibilities:
a) modify the source. search all files for feed_freq_hours, change all occurrences to feed_freq_minutes, and fix all references (eg. instead of $feed->feed_freq_hours * 3600, you will have to use $feed->feed_freq_minutes * 60). you will also have to rename the feed_freq_hours column of the [table_prefix]feeds table. good luck!
b) (ab)use feed_freq_hours for minutely update frequencies by using fractional hours: eg. for an update every 10 minutes, enter 0.1666667 (a sixth of an hour). this will need a change of the feed_freq_hours column from int to float, eg. ALTER TABLE [table_prefix]feeds CHANGE feed_freq_hours feed_freq_hours float.
you might be better of submitting a feature request to the author.