Turning Monthly Data into Daily Data

Hi,
Can someone please help me? I'm in a real bind
I have 100 years of monthly datat that I want to compare to 100 years of daily data. Is there any way of automatically adding for example another 30 rows to January 1900 and pasting the values for January into each row and so on and so forth for the other 11 months?
I know there'a a fair bit on compressing data into monthly from daily but obviously not much the other way round so any help would be very very much appreciated.

Thanks

I appreciate the suggestion, unfortunately it won't work for the analysis I want to do.

It revolves around the implementation of a policy to divert water from a river based on a daily rule eg. X% of flow each day passes that will change depending on storage levels in reservoirs which is modelled on a monthly timestep. In dry periods it might be Y% diverted and in wet Z%. So basically I want to see what days will be below storage level thresholds and the impact that will have on the amount of water passed/harvested if it was implemented historically. Hope that makes sense and if anyone can help, that would be much appreciated.

Thanks