Sounds like you want (or need) to switch to a “push” configuration where the slaves push their data to the master instead of the other way round.
If so, you may want to read the article about distributed monitoring and especially the section 4. Livedump and CMCDump.
Basically the slaves run cmcdump regularly (every minute, every other minute, …) and copy the resulting data to the master site where it is then “applied” (imported, so to speak).