I am doing a shielding experiment, where I wish to shield against frequencies of 0.5 - 100 Hz. For this purpose I have chosen Mumetal. Unfortunately Mumetals initial permeability has a dependency on frequency, meaning after around 500-600 Hz the initial permeability will plummet towards 1.

My question is this: what if the mumetal is irradiated simultaneously with a high frequency (above 600 Hz) signal, and an ELF (0.5-100 Hz) signal, will the ELF signal be able to penetrate the mumetal (Assuming that it cannot on its own), or will the combination of the two create a new unique signal that does not classify as being between 0.5-100 Hz?

Not sure what you mean by 'initial permeability@ but here is a learned article comparing mumetal superalloy and aluminium down to 10Hz.

 

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1139&context=theses

 

Your proposed combined signal will be differentiated (filtered) by passage through/reflection from the shield.

The calculations in the linked article will help estimate the extent of this filtering.