You can not show leading 0's in the spreadsheet (Library view). At this place, they are a floating point number (the templates convert numbers to text)
However, you CAN use leading 0's (or spaces) in a Template for TRANSFERS
Code:
{series_index:0>6.2s}
would give you 003.45 as the series index (3 before, the . and 2 after = 6)
I have my (Library view) series index set to (always when there is a decimal) display 2 digits

for the life of me, I can't find where I did this YEARS ago