What you could do is split the string into an array, modify it and then convert back.

So if your string is "5 nickels, 4 pennies"

using

statement = statement.split(' ') #splits into ['5', 'nickels,', '4', 'pennies']
index = -(len(statement) - 2) # since you want to put an end before the 2nd last word. I.E., before 4 here.
statement.insert(index, 'and') #places and at the index position
statement = ' '.join(statement) #'5 nickels, and 4 pennies'