you need to use the Office RecurrencePattern Object
see msft docs which represents the pattern of incidence of recurring appointments for the associated AppointmentItem
object.
the object has RecurrenceType (olRecursDaily, olRecursMonthly, ...)
which have themselves properties (Interval, DayOfWeekMask, DayOfMonth, ...)
with this information you can build the string defined by rfc5545.
one example implementation is available see : sourceforge otlook2ical