I've found a dirty hack for current version mopub-ios-sdk(3.9.0). You can post your data(repeat value, and fixed path) on your server and then work with methods from class MPClientAdPositioning.
- (void)addFixedIndexPath:(NSIndexPath *)indexPath;
- (void)enableRepeatingPositionsWithInterval:(NSUInteger)interval;
You must independently calculate indexPath for your UITableView, where you insert your native ads. Here you need these methods:
- (NSInteger)numberOfSections;
- (NSInteger)numberOfRowsInSection:(NSInteger)section;
It was helpful for me. Maybe someone know better solution?