If a Musi has excess hissa amad built and would like to apply that excess to hissa Jaidad, is this allowed? If so, how would a Musi proceed as this can not be indicated on schedule C?

Yes. In this case, the Musi must write to the National Wasaya department USA stating that they would like to transfer their excess amount from Hissa Amad to Hissa Jaidad. The National Wasaya Department in collaboration with the National Finance department will transfer the requested amount to Hissa Jaidad and will notify Markaz.