Custom Periods in SOP (planning periods)

Dear Gurus,

i wanted to use the period indicator K in the configuration of an information structure at "MC7F".

The system does not allow for me to have custom periods (as allowed in demand management using the planning calendar) and does not let me save whenever i make it K, it only allows weeks or months..please guide me how it would be possible for me to use for example:

A week with more than 7 days and an year having ONLY 48 periods not more than that (standard system FIXES all weeks at 7 days).

Best Regards,

Danish Sherwani